As far as the irregular periods go, it will be very hard to know if you could have gotten pregnant, the only thing you can do is to wait a while and test. Its only been a few days so it'd be too early to test now. Also, the brownish discharge could be either old blood or maybe you bled a bit since it was your first time? If he didn't "finish" it's very likely you didn't get pregnant, but there is always pre-ejaculation, so there is a slim chance. Best thing to do is wait, and in the future if you don't want to be pregnant use a condom or birth control. Good luck.