Most Helpful Girl
There's really no easy way to bring it up. You'd potentially be accusing her in some way. And no one likes that. But if she's a good significant other it should be able to be brought up and discussed about like adults. but if it's bothering you that much you should bring it up as nicely as possible. You deserve answers. And if she does have something going on with that other guy it's just cruel to perasue that while your trying to make things work. If that's the case she shouldn't lead you on. She shouldn't be able to have it both ways.
Most Helpful Guy
Be honest and direct. Ask her about it when you're together, not over the phone. You have a right to be concerned.