What are the reasons for hitting a plateau?

just keep going, you wont lose a huge amount every week.

QFT

larger losses are expected early on in weight loss as your body adjusts to the new diet (usually healthier habits cause you to drop alot of water weight as you decrease your sodium and carb intakes with the new calorie restriction) You will fall into a slower loss rate that is actually fat. Sometimes the losses are not linear and one week you might drop 0.5 lbs when you should have lost 2 and then the next week you will drop 4. These things just happen, especially for women where hormones can largely effect water retention as well, masking weight loss for short periods of time.

Usually a plateau is considered more than 8 weeks.

on a side note make sure you weigh all your solid food with a food scale and select appropriate entries to keep your diary as accurate as possible. Log everything every single day. You have to keep your expectations realistic and your diary is the only thing that can tell you what you should really expect.

http://www.myfitnesspal.com/topics/show/1234699-logging-accurately-step-by-step-guide

Starvation mode is a bit of a myth metabolic slowdown does occur in vlcd over an extended period of time. But it's not really significant enough to dramatically slow weight loss. I know your doctor told you not to exercise but I recommend taking up walking. Get yourself an mp3 player or something and go out and walk in the sunshine for as long as you can. When you are heavy walking is an extremely efficient calorie burner and pushing the amount of time and distance you can walk will get your heart ready to deal with the real workouts when you come to them.

I don't know if these numbers hold true for you but I burn about 700 - 800 calories in a 2hour walk. Not that you can probably do 2 hours straight right off the bat. But I started at 338lb's which is not so far ahead of you And literally the heavier you are the more you burn walking. And the more it strengthens your muscles too. You would be surprised how quickly this process can happen.

Remember fat people have good musculature by necessity. It's needed just to move when you carry so much extra weight. So I say turn your weight into a silver lining. Walking will improve your leg and core strength and your stamina. You might be surprised to know that the people that have on average the largest lean mass in the world without a doubt are Sumo wrestlers. Not that I'm calling you a sumo wrestler but if you use what you have you will have an easier time of things. The military has it's recruits climb hills carrying heavy bags for a reason. Well us heavy people don't need the heavy bags XD.

For me, I noticed that I'll lose 5-10 pounds for a couple of months and then not lose anything for 1-2 months then lose another 5-10 pounds, etc etc. Before, I would give up once I hit that stall and then gain back the five pounds and then some (this went on for years leading to me being the weight I am). It's only been within the last couple of years that I've really learned this is how my body is. I don't fight it anymore, but I don't give up either. During the months where I don't lose, I still control my eating and I'll still exercise. As long as I don't gain more than a pound or two during those periods I don't stress about it (or try not to).

Just keep on trucking. Congrats on your loss so far!