If it's been two weeks since the incident I advise she take a pregnancy test. Not because I necessarily believe she is pregnant but because you both seem worried and this will help relieve that worry.

If her periods have been irregular in the past... That is likely why she hasn't started now... Even if they were regular the past few months... Irregularity can cause that. I know someone that tends to be pretty irregular but occasionally she will have a few months... Maybe even as long as 6 months... Worth of what would be considered regular.

After you all take a pregnancy test it might be wise to look into contraceptive... Even something like condoms... Birth control is a tad bit more effective but if you all don't have access condoms will be fine. You should definitely invest in condoms if you two have a desire to take your relationship further.

I think it might also be a good idea for your girlfriend to go to the doctor about her irregularity. There are numerous causes for it and it might be good for her to get some help figuring out what is causing it.

Best of luck.