Unsure over potential girlfriend

Basically, been texting this girl for maybe like 6 weeks or so, finally met up and went on a date and it went well. Told her I had some feelings for her and she says she likes me but didn't go to the same extent as me. She told me she was dating another guy but he had a gf, so in my mind they had stopped dating. Over the next couple weeks I saw her a couple more times, with her coming to chill round mine, haven't had sex yet but looks like it'll turn into something between us. Then I get messaged asking me what I want, I told her I like her a lot, she replied by saying she's dating another guy. She asked us both what we want in terms of **** buddy or relationship. I said relationship, the other lad said **** buddy... and she says she would pick me between the two. However I didn't reply in a convincingly happy manner (as in my head I was the only one she had been seeing and we had become a lot closer recently). My problem Is kinda feeling abit lied to and having abit of suspicions in terms of her banging this guy whilst we've been quite close. However when she's been chilling at mine, she seemed reluctant to try anything, as I tried to initiate sex. Plus I don't know what she would've done if the other guy had said he wanted a relationship.
If u were in my position would u feel abit annoyed and confused with the situation, or am I overthinking/reacting and should just be happy the girl I like is picking me over another lad?