I would say that your chances are extremely low. Yes there is sperm in "precum" but you were also on your period which is your body removing the lining of the uterus and expelling the unfertilized egg. I know that you probably already know this but it more than likely got out. Even if it was "fertilized" the chances of it getting out before implantation were pretty good.
I would strongly urge you to get on to birth control now, accidents like this can happen at any time. Also, in many peoples opinions once your boyfriend enters you however slight, you are no longer a virgin. Even if your hymen was not broken.
Best of luck to you in everything that you do,
__________________ Herniated L5-S1 in 2005
Herniated L2-L3 in 2008
Dynamic stabilization L2-L3 in 2008 (failed-spondylolesthis of L2)
PLIF L2-L3 in 2009
PLIF failed to fuse