Been on Birth Control for over four years and all of sudden I'm spotting and cramping between periods?

Been on birth control for over four years and I just starting bleeding and cramping between periods? I am sexually active and have been for almost a year, and I mainly rely on my birth control for my main source of protection and have anal sex most of the time though we do engage in vaginal sex at times as well. Two to three days after I had my most recent period had ended, I started having painful cramps around the back and pelvic area, which lasted about two days. Then, four days later I started spotting which was light pink at first, then as it progressed it became heavier and has been going on for another four days along with some mild cramping as well. I took two pregnancy tests as soon as the cramps and spotting had started and both came out negative. I take my pill every night before i go to bed an rarely ever miss a pill and have not missed one this past month. However, could this mean that I could somehow experiencing early signs of pregnancy such as implantation? or could this simply be from my pill because i have been on it for so long to the point where my body is changing, therefore, my hormones are too? Or could this be a serious health issue?