What Causes Very Light Periods For 4 Months?

I am not sexual active, but this past January i was fingered. I had my period the end of january. Then in February my period was lighter. I took a test to see if i was pregant it was negative. A few days later i got on birth control and of course had to take another test, which was negative. It is now may and my period is still light. I feel as if u am pregant, but i get my period every month, but its not how it use to be even in February before i got on BC. Please help ive been stressing about this for the last four months.