off schedule and unwelcome

I've been on the pill for almost four years so I usually get my periods like clockwork and can guess them to the hour. But the last few months they've been coming at different times. For the last few days I've been having a lighter one but the discharge is blood mixed with something black. It's also causing incredible cramps especially during sex. It may be unrelated but I've been bruising incredibly easy lately. I've had blood work, biopsies and a colonoscopy done within the last month and everything comes back clean.