of course you should be worried. pre-come can get her pregnant. it only takes one sperm to fertalize the egg,and pre-come has sperm in it. you also wont know that you had this,because you cant see it,but you would have had it.it doesnt matter if it was the day after her period,some people ovulate then some even ovulate during their periods. your sperm can survive in her uterus for up to 7 days,so if she ovulates 7 days after you have sex your sperm can still fertalize her egg.
get her to take a test if she misses a period.