If you have been taking your pills properly, then I highly doubt you are pregnant.
You can actually psychologically cause your symptoms-- or they could be because of something different. I found that if you think about your symptoms a lot, they seem to get a lot worse.
Like I said, its very unlikely you are pregnant. Something else, either less or more serious, could be going on. Take a HPT, though it will probably be negative, and then maybe make an appointment with your doctor if your symptoms continue. Constipation and the cramping you are feeling could be due to an intestinal or stomach problem. I'd say wait it out a little bit, then see your doctor if things don't get better.
Good luck!