12 day period on the pill?

So about 14 days ago, i was going to visit my boyfriend in Dallas. I was at the end of my pack of birth control but couldn't get the next pack for 2 days, but i didn't want my special girl friend to be visiting while i was there. I also get really bad cramps, so I've just stayed a new pack before to avoid my period. I figured my period would just be short. When I got the pack and my period started, but went ahead and started taking the pill. I've had my period for 12 days now, when usually i only have it for 3-4. Has anybody experienced this or know what's going on?