Wow, was half-expecting a "Pet Peeves" style thread when I saw the title.
Has she offered any explanation as to why she lied to you initially? Have you had any issues previously with accusations of infidelity?
I ask mainly because, had she been 100% honest from the start with the information you've since discovered, I personally wouldn't have a problem. The fact that she felt the need to conceal it strikes me as being odd.