What Causes Delayed And Short Periods, Back Pain And Vomiting Sensation?

me and my girlfriend had unprotected sex roughly 2 weeks ago, i'm pretty sure i didn't pre come inside her but i could be wrong, a few days ago she got her period which ended up being a few days late, after her period came, it only lasted 3 days, she said she had fresh period blood come out.....she has been telling me her back been hurting about yesterday and she started to feel like she is vomiting this morning......im scared and i hope she isnt pregnant yet