it's been thirty days... Chance of Pregnancy?

Additional info: I got my period in December of 2007. I am fifteen, turning sixteen in two months.

I had sex for the first time on May.26th. I got my period May.27th and it lasted until the 31st (it usually lasts three to five days). I again had sex on the 31st. My period is very irregular. Sometimes it comes every 20 days, sometimes it comes every 28 days. I lost it from September 2010 - December 2010 due to a eating disorder.

My actual problem: I haven't gotten it yet and it's been thirty days. My last three periods didn't take longer than 23 days to arrive, so I'm getting worried. I have been getting headaches (though that could be because of dehydration) and some vaginal discharge. My boyfriend used a condom both times and was probably only half in as well as was only in me for two or three minutes. My main concern is pregnancy. I'm aware that there is a possibility every time I have sex, and I am getting worried because of my missed period.