Can my gf be pregnant ? and is it mine ? lol?

me n my gf had sex on the 3rd of march... it was the first day of her period... i ejaculated inside her... the next month she got on her period and we had sex again but this time i didnt ejaculate inside her, but precum must have been inside... so march we had sex, den the nex month she came on her period n we had sex again without coming inside this time... n then this month (may) she missed her periods... overdue for a week+ n she has had a few tests done and its positive.. is their a way to get pregnant how i have just described it ? and is it mine or has she been unfaithful? thanks.