Unfortunately, unless you have some corroborating evidence (like a text message of her confirming it), all you might have as evidence is your own testimony. Whether law enforcement proceeds based on that or not is up to them.

If you were merely inside her, but did not ejaculate, then there is almost zero chance she is pregnant. However, if she does get pregnant, takes it to term, and you're the father, then you will have a legal responsibility for that child regardless of the fact that conception was without your consent.