If the pregnancy
tests are negative and and if you are getting periods there is no possibility of pregnancy.
There are two reasons why your belly may be looking big.
1. What is your age? May be you are putting on belly fat. In that case, you need to exercise well.
2.Please get an ultrasound
done. Sometimes the presence of ovarian cyst
or fibroid uterus( smooth muscle
swelling of uterus) can cause the belly to look bigger.
I hope I have answered to your satisfaction.