Well, if you slept with both of them within i'd say a week.. It might be hard to figure out who the father is.. I think after you're so far along, they can actually do a paternity test (with the baby still in the womb). I just wouldn't tell either of them that they are the father until you know for sure. You could run into problems with them if you claim one is the father, then it turns out the other is..

Since you didn't say the amount of time in between of the two guys, try to figure out your days. As that's all you can really do if it's that.

Most likely though, the father could be your current boyfriend since you didn't use any protection with him and did with your ex. But that's still not 100%.. Good luck.