I agree with the general opinion thus far, but I more agree with Kunoke's concern with you knowing absolutely what is going on for sure.
Communication is very important, and however unlikely it may seem to you, nothing serious may be happening. If you are the jealous type its actually more her problem then your problem as she's got to contend with you and you may be driving her away. Consider what has put her into these episodes and try to get to the root of the matter before making up your mind.
I think it would be a horrible shame for this relationship if it was ended because of suspicion and doubt alone, with no proof to back anything up. Consider how she feels about this. From what you have said so far, this presentation of the situation seems rather one-sided. Without knowing what she's doing or how she feels its very hard for any of us to truely give you worth-while advice.
I can only say now however that it may very well be premature to end the relationship on little more than several white lies. Did you ever consider that maybe she didn't want to tell you because she thought you're the jealous type and wouldn't be able to handle it? Its hard for people to admit that type of thing, and I'm not labeling you as jealous, but you may want to consider why this is truely bothering you. Surely when you find out, you'll understand her point of view in keeping it from you.
Ultimately, a relationship is built on trust and companionship. If she's giving you additional reasons to not trust her and she's drifting farther and farther away as a companion, then perhaps you should reevaluate this relationship and either work to salvage it or be rid of it.
Professionals actually say that when couples don't argue or challenge eachother at all for the production of a relationship, its a sure sign that there already is cheating going on, or there will be, as the relationship has become so complacent and taken for granted and not built up, that infidelity is just abound.
If you and your girl have an otherwise healthy relationship, then I see no reason for you to be so upset. If you can't trust her with an ex, then you don't trust her period. Trust is absolute. Theres no such thing as trusting someone just a little bit. You either trust someone or you don't.
Again, examine your own mind, thoughts and conscience, and perhaps the fault lies there as well, and in addition to anything she may be doing that you feel isn't right.
It takes two to tango.
Talk to her, and explain how you feel and be sure to ask her how she feels. Honesty is not only the best policy, but it can redeem so much and begin a true healing when hurt and pain has been dealt.