Condom keeps falling off..?

I've not had sex for a couple years until recently. Me and a girl I've been dating have started having casual sex. However, 2 times in the past couple weeks, the condom has fallen off. This has NEVER happened to me before, so I don't know why it's happening. I'm a little over 7 inches long and 5.5 in girth, so I don't think it's a size issue. I know the first time I wasn't fully erect because I was having issues with a medication I was taking, but last night it was fine. The lining of the condom was even indented on my shaft. The condom was fully extended, as well. We do not use lube, but the girl I am with is always well lubed, naturally, after some foreplay, so it's not an issue there either. I really don't know what to do. She's not on birth control, yet, and neither of us want a kid at this time... lol. The only thing I can think of is that she'll start pulsating and get really tight after we go for a bit and it might get pulled off from that, but I don't think so because, like I've said, this has never happened to me in the past. It's just a little embarrassing for the both of us, and mood killing.