if your blood tests are coming back negative and you are having your period then you are NOT pregnant. maybe your just gaining a little weight, my advice- get a gym membership and watch what you eat if you think your getting chubby
Maybe get an ultrasound done?? Or think about your diet/excerise?
Or if your under stress, that could be a part of it. I do know the feeling hun. There are many reasons woman gain weight. With my grandmother it was her thyroids (sp?)
I would go to the doctor and explain your situation with diet/exercise, stress level, or chance of pregnancy. I've watched shows where some women didn't know they were pregnant, until they were about 4-5 months. Anything is possible! Good Luck Hun!!