Self Judgement

Are you logging how many calories you are consuming and burning? Exercising and eating the 'right' food will not contribute to weight loss unless you are burning more calories than you are eating. If you are not paying attention to how much food you are actually eating it is easy to eat more when you start exercising more.

This thread is a great read to help you get started in the right direction.
https://community.myfitnesspal.com/en/discussion/1080242/a-guide-to-get-you-started-on-your-path-to-sexypants

Are you weighing yourself at the same time of month as last month? Do you have a regular menstrual cycle? Depending on where you're at with that, it can account for a skew in the numbers. There's also merit to the idea that you weigh at the same time of day, preferably after morning bathroom business but before breakfast, naked, at the same time of month. If you can tick off each one of those as a yes, then I would start looking at other things... Yes, a food scale is important, as is considering the potential of physical activity rendering changes that produce water retention. However, I would start looking at it like this...

Comparing yourself to your boyfriend is going to be a problem off the bat and a hotbed for resentment if you're not careful. Men and women's bodies react differently; you should compare you, the whole you, to who you were a month ago, and it's not just about the numbers.

The changes you've described are not relative.
Your boyfriend enacted a weight loss plan of cutting out a high sugared food that I'm guessing he drank daily prior to this. If all other things stayed the same and he just cut Mountain Dew and replaced with water, he took his normal daily calorie intake down by at least 600 calories. 600x7= 4200 calories a week cut, coming out to a little over a pound a week.
You could have done the same thing, if that's what you were going for. However, your changes are more what I would call a healthy lifestyle change. You're not focused on weight loss alone... While that might be your primary goal, your actions are not exclusively about weight loss. The saying is, with varying numbers, 80% diet, 20% exercise. So if it was just about losing weight, your focus would be on diet changes first. However, you've also enacted a gym habit of 3-4 days a week, which is an increase of your burn by probably what, 800+ calories a week? If you were sedentary before, this is a massive change and your body is most definitely going to retain water... I would argue that it's even more likely if you're not balancing your calorie cut and your workouts.

That is, if you're using a calorie cut like classic MFP, you've got your weight loss portion covered. Adding exercise onto that total means you should be eating more on the days you work out. Are you? Why or why not? There are different camps for how to work the calorie balance out effectively, but if you dropped your diet by 1000 calories a day, that's plenty enough to spur on 2lbs/week... and if that cut has you down around that 1200 mark, you have to be a bit careful about how far down you dip, especially being diabetic.

I don't think diabetic medications are going to be inclined to cause sudden weight gain unless you just started taking something new, and even then, that's iffy. Most diabetes medications would come with a sudden weight loss concern; possibly a difficult with weight loss in general with medications like Metformin because they work for regulating sugars, and in ways, loss requires your sugars to get low/insulin to spike high. That's why weight loss is a sign of diabetic onset... The body goes into super-insulin creation as the illness gets going. However, if you're certain that a new medication has caused weight gain of that amount of the last month despite doing "all the right things" I would suggest talking to your physician.

So blah blah I know, I talk a lot. My point mostly is that we have to make sure we are making the right comparisons at the right times. It's highly likely that your boyfriend will always have an easier time losing actual weight than you will, males are notorious in that way. If you know you are judging yourself, then it's time to give yourself permission to... well, not. That judgment is a choice. You can either look at the scale and say, all those beautiful, healthy things I've done for myself mean jack because I gained 5lbs... or you can look at yourself and say, I am so proud of myself for keeping up my new gym habit and sticking to a diet my endocrinologist would love! I'm not going to worry about those 5lbs because for all I know, I just haven't pooped recently enough. I'm not going to worry about it until I know something is really wrong.

It's not to say that a small gain can't be something wrong... but I mean, what indications do you have that there IS something wrong other than the number under your feet? How do you feel? Have you made progress in your gym routines? Sleeping better? What about your sugars? Try to think about the intangibles... because while the scale victories are nice, the no scale victories are the ones that save a life.