What Causes A Delayed Period With Stomach Swelling?

hey, just started going out with this girl 3 weeks ago and we havent had sex but we have played with each other. but she made me wash my hands before i did anything, she was due her period 6 days ago and it stil hasnt come, she said her stomach is swollen but there still seems to be no sign of it! what could cause it to be late? she's kinda worring so i thought id look round on the net about it!