Possible cheating advice, found strange clothes in her laundry.

Can anyone give me some advice or has any experience with this, or any women that have some insight, maybe even if youve cheated on someone? I have no idea how to handle this, things have felt strange recently and now this. How do I even hint at this or find out or just bring it up even?

So recently the last to days my girlfriend hasn't texted me back all day until about 9:30 at night, then again at midnight and 3am. I thought ok that was weird but she called me as usual before bed. Then the next day she goes to saying she misses me and wants me to come over, I spend the night with her and she leaves for work the next morning, I go to grab my jacket and I notice she has worn a nice black shirt she usually wears when we go on a date and underneath some other clothes in the hamper is her sexy black lace thong. It was heavily worn, by that I mean she definitely enjoyed herself in them. But I know girls discharge but it just doesn't add up. she hasn't worn anything like that for me in about a month or even worn something nice on a date. I know she usually wears thongs with leggings but this just seems too coincidental to be her sexiest one. What should I do about it or how is it possible to even bring it up. I know I shouldn't have gone in her clothes and I realize its untrusting to do that, I just saw one and then couldn't help it, it seemed so off. How do I bring this up or even start to talk about this subject?