not trying, but not preventing.

So my boyfriend of 4 years and I have been talking about kids lately, saying if it happens it happens. We aren't trying but we aren't using protection either, I haven't been on birth control in about 8 months and we had sex A LOT this weekend. (My birthday weekend so we went a little crazy) and turns out I was ovulating one of those days. Does anyone know what the chances are that I am going to wind up pregnant?? Just a little curious.