Doing something wrong here

You must not be eating at a deficit. It sounds like you need to reevaluate your intake of calories and perhaps invest in a food scale so you get a more accurate calorie count. It's normal to gain a bit of water weight when you start working out, but that's just a couple pounds, not 42.

This is beyond perplexing, we are the same age, doing the same cardio and probably close to weights (actually you are doing 2-3 times more a week than me) I started at 300 pounds and went down 26 in 6 weeks. Admittedly I am eating under my calorie limit. but based on the two entries in your diary and your info in the post you are too. Something is seriously off....I put your calorie deficit at 2500 a day (probably an additional 500 more with the gym work you are doing....that's at least 5 pounds a week you should have lost.

You are not being completely honest with your calorie intake. You have to log and measure your caloric intake every single day.

Exercise is great for body health but, for weight loss, the most important thing is that you eat fewer calories than your body uses in a day.

At your weight, you can retain a lot of water and the amount can vary. How is your salt intake? Once I started keeping my salt under 2400 mg per day, that really cut down on the water retention.

Pre-packaged foods do tend to contain a lot of salt so watch for lower sodium versions. They can also weigh more than they say they weigh. I had a Lean Cuisine a few days ago that had nearly 100 more grams of food (mostly rice) than the package weight said so it had more calories.

Double-check your portion sizes. The package may look like it's a single serving but actually contain more than one serving.

Double-check your MFP database items to make sure they have accurate information compared to the package or USDA info for the food.

Start today with recording everything that has calories into your food diary right before you eat it (or before you leave the house with it) if you can. If you get into the habit of logging before you eat, you can look at the entry and perhaps change your mind about whether or not it fits into your day and if you might want something else instead.

People are suggesting very good things here.

You should try weighing all your food, even prepackaged food, for a while.

1) Remember to include all alcohol.
2) Don't count weightlifting calories burned for eating.
3) For other exercise, don't trust the burn rate it gives you on the machine. It's not correct. And even if you punch it into MFP, it's up to double what you burn. So divide it in half.

If you follow the advice of the people above as well as in the stickies for say 4 weeks and your weight is still going significantly up, go see a doctor, pronto!

I see a major red flag. If you think you're eating 900 calories a day... even if those packages are off by 25%, you're still around 1200 calories a day... and to gain 42 in three months would mean you're eating around 1600 calories more per day than you burn through daily life. And let's assume you burn 2000 (you probably burn more though).. then you're looking at over 2600 more calories than you think you're eating. And that's quite a bit... I think you would know where it was coming from.

Just to make sure, are you counting all your fruits and veg? They are not free - they still have lots of calories. I just want to make sure, because that is actually a decently common misconception.

And also keeping in mind to use the right database entries (i.e. are you weighing your meat raw or cooked, pasta and rice, raw or cooked?).

I honestly have to ask... have you tried filming yourself at night? What if you sleep eat?
It could be your thyroid also. Or something.

When I quit tobacco, I went from 142 to 188 lbs. in five months. Why? (1), I adopted the attitude of "Anything to not cave.", so I ate. A lot. Of stuff I hadn't eaten or drunk for years at that point. Whole large pizzas in an evening. Six packs of beer in the same time frame. Cans of macadamia nuts and sunflower seeds commuting. Sitting on the computer interacting with my quit group/forum every spare minute instead of exercising. (2), It's a known fact nicotine upregulates metabolism. Quit, and your metabolism takes a nosedive. For some, at least anecdotally, it also causes their thyroid's activity to nosedive as well.

I am on a fixed food budget. I have been for several years now. I cook 99% of what I eat. As in actually cook vs. from boxes, too. I eat the exact same things for weeks to months at a time.

I still got fat because I refused to weigh all my food and count calories. Since I have, I have lost weight. For the first time since November 2012. In fact, at a faster rate than I did from July 1-December 1 2015 when I was minding portion sizes but doing neither. Like, at double the rate I wanted.

I don't exercise. At all.

Put your insistence to the contrary aside, just for 30 days, and weigh all your food, count all your calories, log them all. Everything. There is no free food or foodstuff. Not even supplements. Try to hit the (reasonable) daily caloric intake you set based on your inputs to the calculator on here more times than you don't in that 30 days. Stay hydrated (i.e., except for the first micturition of the day, all urine produced is pale yellow in color). Also, don't overestimate how many calories you are burning exercising. If anything, underestimate. Eat either none or only half of them back.

And at the end of that 30 days, see where you are at.