My girlfriend thinks she's pregnant, but I doubt that she is. She's not sure when exactly her last period was, but she thinks she's several days late. She had a day or two of sickness (vomiting) about 10 days ago, and now is tired and achy. I believe she is tired and achy because of her stressful schedule, and the fact that she plays soccer. She hasn't had more than one symptom at a time, and they haven't been anywhere close to severe.

Some facts:
Never have had sex without a condom.
Had sex once since her last period, but I didn't finish.
Sex was a day or two after her last period

I wish I knew more so I didn't have to ask. We need your help. Thanks.

The only way to tell is a home test or doctors blood test.
Stress can do a lot of weird things to the body and delaying her period could very well be due to that.

Agreed. A home pregnancy test would be the way to start. Get a good one, as some of the cheaper ones are not that accurate. This would set both your minds at ease.

Condoms even when used as directed are know to have failure rates from 3 to 5 percent. When used as most people do, many do not use correctly, the failure rate is average at about 10 percent.

Condoms help a lot but are not 100 percent effective, many people get pregnant using condoms.

Don't have to finish, what many call the pull out method is not effective at all. There is plenty of sperm in the early fluids to get a girl pregnant.

Women ovulate at different times, so unless she was tracking her fertile times it is impossible to say on timing.