You guys should buy condoms that are a different brand or size, because a condom should not slip off.  If you are on the pill and have taken it as directed, the chanced of getting pregnant are just 1%, regardless of wearing a condom or not.  Also, since you say that his penis didn't even really enter you, the chances are even more slim.  Keep being careful like you are, buy better condoms, and don't worry.  I think the chances of you being pregnant are almost none.