What happens to your body when you stop taking birth control?
There isn't really a waiting period to bring your body back to its baby-making ways. You may get pregnant right away, but bodies do vary. Any symptoms you may have had before taking the pill could return, like PMS symptoms, acne and irregular periods. It may take a few months for your body to get used to normal hormone production, so periods may be irregular. You may also feel yourself ovulate.
Why is it so difficult to lose those last 10 pounds?
It depends on how much you just lost. If you dropped 40 to 100 pounds then don't lose perspective of the accomplishment you just achieved. The last bundle of pounds is harder to drop because you burn fewer calories in daily activities when you are slimmer. Amp up the intensity of your training so you cook those calories more efficiently. Try HIIT (high intensity interval training). Stay as physically active as possible. Make sure you're burning 400 calories a day so you can have that deficit and lose the weight. A fitness tracker may help. Don't expect overnight results.
How long should I stretch before and after working out? And what kind of stretches should I be doing?
Be careful with your stretches. Most people would do well by loosening up with high kicks, back kicks, hip circles, simple squats against the wall and light lunges. Brisk walks can help with stretching. Don't do anything radical with jerking movements. Dynamic movement counts more than anything else.