sometimes birth control can mess with your hormones. I would ask my doctor if you could get an ultrasound.
Yes if you stress over it too much you will start to actually think you have the symptoms and sometimes you really do have them. If you are pregnant you would have to be a few months along to actually have a larger tummy that is harder. You might want to consider having a blood test because sometimes the urine tests will say negative when your pregnant. I had to end up taking 7 urine tests before I found out. There can always be a false negative but never a false positive.
you can have thoses symptoms because your mind is on it. i.e. the symptoms are brought on by you if you are still worried get the dr. to do the blood test