You wouldnt be so big so early, its in your head.

If your preg. Then you may be further along. But I was in maternity before a full 3 months with my 2nd. I wasn't even a full 8 weeks and my lower tummy was already hardening. My belly got huge. The best bet is to call your doctor, tell him/her that you have taken a few preg test and they are coming out negative and so on. They may do a blood test or ultrasound. A girl I work with knew she was pregnant but took 2 hpt cause the 1st came out negative and she knew she had to been at least 5 weeks. She took another a week later and still negative. Went to her dr's and they did a urine-negative, so they did her pap (planned to do anyway) and said she was definitely pregnant. By this time she was about 6 to 6 1/2 weeks along. Your cervix turns a bluish color when your pregnant.
But also like the other writer said, it could be in your mind. Sometimes when people want to be pregnant so bad they can actually start thinking they are having symptoms and not even realize they are doing it. It's mind over matter!!
Good luck though! Hope it comes out the way you want.