Didnt pull out.

My boyfriend and I have been together for 3 years and we use the pull out method most of the time, but sometimes we use condoms. This past Friday however we didn't use a condom and he didn't pull out.  But I got my period yesterday (a little early) and its lighter flow than normal with some cramping in areas I'm not used to. My periods/symptoms are very regular so I can tell when they are different...any ideas of what this could mean? Thanks in advanced!