Weird discharge?

I don't get a normal period and haven't had one since I got my mirena put in last September (2014). Yesterday I noticed I had horrible horrible cramps all day and this morning I had like yellow/pink/white discharge. I haven't had it since then (must've happened overnight) and today I have just a few cramps here and there. I had spotting (closest thing I've had to a period) on the 16&17. I have a mirena so I doubt I'm pregnant but what could cause that? (I had sex consistently for two months before the 23rd with an ex if that helps any)