Long strange periods, could I be pregnant or is my birth control acting unusual?

[JayLovely21]

Last month I had a 13 day period that was very strange. It lasted extremely long my normal period is about 7 days long it was also very light. I only needed a liner. Now it's been 5 days since then. For the last two I have been spotting. I took two pregnancy tests both of which have come up negative. I got the depo shot in October. This would be the month it's leaving my system. My last encounter with unprotected sex was also in October. Twice during the first of getting the depo shot. My body has never done this or been this abnormal with periods. The last time I was on depo it took away my period all together. I don't know what to do and I don't know what's wrong can someone please help? Has anyone ever experienced this?