The first thing that needs to be ruled out here is pregnancy
. I understand that the test is negative but it is not a very sensitive test. Please get the serum beta HCG
test to rule out pregnancy.
scan of the uterus will help us in confirming the actual cause viz-a-viz pregnancy, periods related issues, hormonal
problems etc. It would be ideal to go for one.
Hope this helps you and put your mind at ease.
I wish you good health.