Could this cause someone to become pregnant?

I am a 25 year old male, and my girlfriend is a 20 year old female. We were toying with each other the other day, and I got some pre-ejaculate on my hand, wiped it off, and then proceeded to toy with her vagina with my hand. I did not wash my hands in between. My girlfriend just got through with her period, and said it's not her time, but we are still both worried that there's some possibility that she could become pregnant. I tried to calm her down and assure her that because I wiped off my hand, it wouldn't happen, but I can't say it with 100% certainty. We are both just a little worried about this, and would like to know if there is any chance, and how much of a chance there is of that happening. Thanks.

Inadvertent Insemination

Inadvertent insemination like this is technically possible, although in reality the probability is low particularly as the pH of the outer vaginal area is hostile to sperm. Also, your girlfriend had only just had her period.

If you are concerned about this, depending on the timeframe, your girlfriend could look at the 'morning-after pill' which can be used up to 120 hours (5 days) after although ideally within the first 3 days. The earlier its used, the more effective it is and is usually available from health centres or pharmacies.