Spotting after Period

Staci
My last period came about 10 days early, but seemed to be a normal one. Yesterday, 5 days after my period ended, I noticed some very light spotting and have had some today as well. There's also some mild cramping.
I have been on the minipill for about 4 months now. I take it diligently. I took a pregnancy test last night; it was negative.
I don't really know what to think. I'm just worried there might be something else going on. Does anyone have any ideas?