What are the chances of pregnancy with a condom?

My fiance and I are definitely not ready for a child. We had sex on Thursday, twice in a row. And then again on either Friday or Saturday. It was our first time. In all, we've had sex probably 4 times. We used a Trojan Ecstacy condom, and we used them properly. I naturally have a weak stomach, and am usually nauseous. I have been nauseous this week. It may be natural or be due to pregnancy. This has been a scare. My period isn't due for a few days, but I'd rather know the chances so I can get a little sleep until then.