Spotting, no period, negative pregnancy test

Phoenix
Just looking for some advice: I'm on birth control (the pill-so I know it's not 100% reliable) but anyway, my period last month was VERY light--almost nonexistent, and only lasted a couple of days (usually lasts 4-5). This has never happened before and my period has always parted on the same day every month. I've been spotting pretty much all month since then and my period was suppose to start a few days ago but hasn't started yet. I took a pregnancy test this morning and it was negative... Does anyone have any ideas/suggestions about my situation?