900 cals okay on days not exercising??

Did someone say, "No," yet?

Oh, they did. Good.

Yeah, no, that's not okay.

No that's not what you should do. MFP is set up with your deficit built in. Your calorie goal is a goal because you're meant to reach it. Also you should be eating back at least half of your exercise calories.

^ This.

I would just like to add in an extra NO, just in case.

I guess the No's have it.

But let us go through all of the maths.

I assumed you were 5'6" you may be 7' but thats what I assumed

So your BMR is 1950 calories, that is if you were comatose you would burn 1950 calories. So if you wanted to loose 2 lbs a week that would be 1000 calorie deficit so 950 calories, assuming you were comatose. Office workers (going to the office fridge I assume) burn between 1.2 and 1.3 times their BMR so 2300 a day, less the 1000 gives us 1300. So the minimum you should eat is 1300 or so on non exercise days and more on exercise days.

You have unrealistic expectations of weight loss...20lb a month for someone your weight is not reasonable at all.

You didn't gain the weight all of a sudden so don't expect to lose it that quickly.

Consistent and substantial under eating is counterproductive to weight loss. You body will start to consume muscle instead of fat for energy...and you may lose a lot of weight quickly for a period of time but you will be skinny fat and very likely your metabolism will be compromised.

Unrealistic expectations are set ups for failure, you become disappointed when goals are not met.

100cals under 1500cals per day is not likely to have a negative effect, however 600cals under will.

Thank you everyone for the advice. According to mfp I'm supposed to consume a little over 1500 cal a day but I set it to 1400 because I know that I will lose more weight and I'm not actually starving myself. I actually want to cut down to 1200 but I'm trying toslowly shrink my stomach. I Weigh 254 lbs and I just want to lose this weight as quickly as I can. And someone my weight should be able to lose 20 lbs a month easily but I know I cant do that eating 1500 cal

Er, no.

"Shrink your stomach?"

Try reading the links already posted and having a little think about what is sustainable.

Well not everyone burns as much as they think. A sedentary person (female) can burn as little as 1600-1800cal daily. An semi active person will burn more than 2000 at least (an hour light like walking per day). If you are hungry drink water and decide. Don't full on starve or eat more because others tell you too. I've been fluctuating between 1000-1200cals minimum a while myself. I find low calorie limits work as everyone surpasses their calorie limits sometimes no matter how high or low. No less than 1000 calorie limit especially with exercise (up it to 1250 with exercise at least). I have researched so much to lose weight and have 8kg lost so far, though I still have 20kg left.
Drink plenty of water, think about your actions and remember advice or not you are who makes your own decisions.

Thank you everyone for the advice. According to mfp I'm supposed to consume a little over 1500 cal a day but I set it to 1400 because I know that I will lose more weight and I'm not actually starving myself. I actually want to cut down to 1200 but I'm trying toslowly shrink my stomach. I Weigh 254 lbs and I just want to lose this weight as quickly as I can. And someone my weight should be able to lose 20 lbs a month easily but I know I cant do that eating 1500 cal

What kind of exercise do you do?

It's not about making the biggest deficit you can to lose as much weight as you can. It's about making the deficit manageable so you don't sacrifice health and LBM as you lose the weight.

Thank you everyone for the advice. According to mfp I'm supposed to consume a little over 1500 cal a day but I set it to 1400 because I know that I will lose more weight and I'm not actually starving myself. I actually want to cut down to 1200 but I'm trying toslowly shrink my stomach. I Weigh 254 lbs and I just want to lose this weight as quickly as I can. And someone my weight should be able to lose 20 lbs a month easily but I know I cant do that eating 1500 cal

As others have said, your expectations just aren't reasonable. You can't "slowly" shrink anything at 20lbs per month. Slow actually means slow. You should aim for losing 1% of your body weight per week to be reasonable. Don't cut further than suggested by MFP, and eat back at least most of your exercise calories.

And here's a good article on why very low calories + lots of exercise is not a good idea:
http://www.bodyrecomposition.com/fat-loss/why-big-caloric-deficits-and-lots-of-activity-can-hurt-fat-loss.html

No. Eat.