Is there another guy in the picture?

Quick Background: I've known this girl for about two months now and we see each other at least twice a week in class and are lab partners. The first time we hung out, she initiated it. Since then, we have hung out 5-6 times mostly one on one, but once we went to a lions game and she met my close friends who she said she loved. The most we have done is kiss, and we kiss every time we see each other now. We hardly ever text and sometimes she'll snapchat me or text me something dumb (funny). As of now, besides seeing her in class, we have plans to go to a big college campus and meet up with friends Saturday for Halloween and have a good time.

So everything was going amazing. Until recently, she's been acting cold/distant. My birthday is tomorrow, and after class last Wednesday she texted me saying that she wants to do something for me for my birthday (which would be tomorrow). I thought it was cool and told her I'd love to do something with her. But the next day (last Thursday) is when she started acting weird. She came over and was acting very quiet. I asked her what was wrong and she said nothing. Then after a while she finally started acting normal. I'm not sure why, but I feel it may have some correlation with this. I told her, Friday a bunch of my friends and I were going to drink a little at my house and go out for my birthday. I told her she can come if she wants too, but i didn't know if there was a point for it too since she's only 20 (which may have sounded like I didn't want her there, but wasn't how I meant it). She didn't seem to thrilled and said she'd text me and let me know, which she never did (unlike her). So later that evening, I snapchatted her twice, and she never responded. Which was also not like her. Then, Saturday night comes around and she texted me asking a question, I responded and asked her a question, and she never replied. Not like her again.

So I saw her today in class, she said hi and asked me how my weekend was and I asked her how hers was. Then when class ended, sometimes we walk to my car and I drive her to her car just so we can talk more. She waited for me (teacher was doing attendance and she got let out early) and then I said what's up and she said she didn't know what to talk about and we went our separate ways.

So four things I think could be the reason to this:

1. She's actually mad at me for the way I acted about my invitation to going out Friday and making it seem like I didn't want to be with her.

2. She may think I am talking to other girls since I didn't want her there and I was texting my mom in class and she may have thought it was another girl (seeing as I never text her really

3. She's talking to another guy and is losing interest in me. I did notice that she's been on her phone more.

4. I'm just overthinking things, maybe she is, and should stop worrying about it.

I see her again Wednesday. Tomorrow's my birthday and I'm just hoping to hear from her and I want to see her of course. We are still going out this Saturday for Halloween parties. She just seems so distant out of the blue and I don't know what to do. I do not want to ask her again because I feel like that makes me look in confident since I asked her Thursday. What would you do? What do you think is going on?

Thanks!