Hello I Am 18 Years Old And Sexually Active. I

hello
I am 18 years old and sexually active. I am on birth control and am using a 28 day pack. I had sex about a month ago with no other contraceptive apart from the pill (which I always take within a 20 min period of the same time each night). I got my period a week after (while on the sugar pills as expected) and it was quite light. Last week ( the week before I am supposed to get my next period) I started spotting and having mild cramps. I took a pregnancy test last wednesday and it was negative. Today I started my period and it was exceptionally light and I'm still experiencing cramping. Nervous that my period was taking so long, I took another test yesterday ( on the 3rd day of my sugar pills) which was also negative. I am concerned and was wondering about explanations