There is hopefully an explanation for everything. Without talking to her your mind is going to instantly come up with the worst possible scenario and from then you'll just be backing it up in your mind.
Like other people have said, the tags were still on the underwear so she's not worn them yet for anyone. It might be that a girl friend of hers bought them for her or that she got them to surprise you with.
She could just be saving the money for a holiday or something? Women tend to save money (generalisations ) because 'just in case'.
Of course, your suspicions could be right and she could be cheating but until you talk to her there's no reason to bring yourself down with it. You've already said you're going to talk to her so I won't suggest it again :p I hope it all works out for you both!