First make sure. Then you have to find out if there are any other guys she's had sex with lately.
If she is pregnant and you are just going to assume the baby is yours, then you have lot's and lots to think about. A little life that will need little diapers and little doctor appointments and a place to live and food to eat......for at least 18 years.

If she's not pregnant, I suggest condoms from now on and any other form of birth control there is. IE the pill, IUD, diaphram, sponge.....

I'm not sure what to say. It's common knowledge that when penis penetrates vagina and ejaculation occurs, there is a chance of pregnancy.