Spotting 5 days after my period. Why?

My period is always normal, I am on birth control and I usually take it every day. I have been on my birth control for 2 years now. The last time I had sex was Nov. 13(we didn't use a condom) and I haven't had sex since then. I started my last period on Dec. 4 and it ended Dec. 9. (This period was a day late which is not normal for me) Then Dec. 14 I started to notice that I had light red spotting that went away after an hour, then came back later at night. I thought this was normal because I have had that happen before. But then since the 14th I have been having redish/brownish spotting on and off through out the day. I have had little cramping but this is not something normal for me. It is now the 19th and I am still having more brownish spotting. I took a home pregnancy test yesterday and it said it was negative. I'm just not sure why I am spotting my period has been normal for the past 2 years. Is there a chance that the test was a false negative? Or could there be another reason for my spotting? Please help me, I am a bit concered about this. Any help would be greatly appreciated!