could i be prego?!?!

Well my boyfriend came home from the military and he had 18 days home. This was our first time together having sex. The first time we had sex was amazing. And we had sex two times before i had my period. I got my period about a week later. And i was relieved because i wasn't pregnant. Then we had sex a few times while i was on my period because there was nothing else we could do and we had to make the time he had home worth it, since he was leaving in a few days. BUT we used a condom everytime. I heard while on your period, if you have sex, you have less of a chance of getting pregnant. so we had sex twice before my period. and 3 times while on my period. it's about a month later. and i havent got my period yet. im 3 days late. and started to panic. we used condoms and were very careful. Does anyone think i could be pregnant or is there lesss of a chance that i'm not? HELP! also, im on a medication that has a side effect of missed periods. And i just increased the dosage of that about 3 weeks ago, im hoping thats why