My girlfriend could be carrying another man's baby

I have been living with my girlfriend for about a year now. I am Muslim and she is a Christian.

She had her period at the beginning of June and during that month; we never had sex because it was the month of Ramadhan.

At the beginning of July, my girlfriend missed her period and after sometime started behaving oddly.

It later turned out proved she is pregnant. I believe my girlfriend is carrying another man's baby.

What should I do?

Asaba