He seems to have just stopped liking me out of nowhere, I'm crushd. What do you think happened?

Ok there's this guy- 'Seth'. Him and I share a mutual friend named 'Marissa'. We're in highschool. In August, we were both at Marissa's birthday party and we really hit it off. we were firting, laughing, and hanging out the whole time. after i left, apparently he asked marissa for my number. i was soo psyched because seth is this really amazing guy. anyway, days went by though and seth never texted or called me. I asked Marissa what was up and she said that he was just feeling shy about texting me, but he wanted to see me again. so marissa dragged me to this charity party because seth was going and he wanted me to go too. we talked and stuff, but after that he still never contacted me. marissa kept insisting that he liked me, he was just being shy (which seems SO out of character). Here we are, October. seth and i have talked a bit through september and we danced alot together at a dance two weeks ago, and i feel like he's always looking at me... but like what the heck. obviously he doesn't like me anymore, right? and marissa gets really distant whenever i ask her about seth, so i stopped talking about him weeks ago. do you agree that seth told marissa something, like that he likes someone else or just doesn't like me anymore? why wouldn't marissa tell me that?! UGH! sorry i'm being so paranoid. i'm a teenage girl, we overthink things.