A girl that I liked (15f) started acting cold after I (16m) confessed my feelings for her... Please help

So I have a huge problem. I started hanging out with this girl I really liked. We weren't officially dating but I think we both knew they where basically dates since all our friends knew (we have a lot of mutual friends). Everything was going well, amazing even. I was super happy. Finally a girl that I really liked and wanted to have a relationship with liked me back ( I have never had a serious relationship before only hookups and girls that I thought were cute so I dated them for a while). The only part left was to officially confess. Now I thought that maybe it wasn't the best idea since it conffesions have screwed me over before but I thought we were really close to being together so maybe it would work. I tried arranging a meetup so I could confess face to face but she was really busy that week so I decided to call her and tell her (I was really impatient). She sounded happy but she didn't say that she liked me back. I chalked it up to her anxiety and shiness. I was happy but then she told me that right now she doesn't feel ready for a relationship and that she wants to focus on school since she some big exams coming up, but if I could wait then maybe we could give it a try. But even if I waited she later told me that she had to think about it since, as I said before, she doesn't feel ready. I accepted it and I told her that I would wait. Also her best friend has a crush on me for a while (I know it's random I just didn't know where I could fit that in). Now the problems begin. It's been a couple of days and now she started acting waaaay colder to me. Really dry responses to my messages and not really wanting to talk to me. We talked about it and she told me that she was just stressed, tired and she just didn't realise that she was being cold to me. After that some of our mutual arranged a meetup (something super short, basically just hang out and talk a bit) and she just straight up denied which bothered me even more since it wasn't just me and her it was a whole group and then she went and hanged out with other mutual friends of ours... So yeah I have absolutely no idea what is going on, how I should approach this, ehat to say or what my next move should be. I have a theory though. One of my best friends is also really good friends with her. Now I have talked to him about this and he told me multilple times that he isn't intrested in her and that they are just good friends (he also has a girlfriend) and I believe him. The thing that worries me is that the girl I like doesn't know that he isn't intrested and she might have started liking him. These are just theories but I'm really starting to get worried. So yeah, I really need help since I was really happy thinking I was finally gonna be in a happy realtionship that is based on more than just looks and hookups. And advice is highly appreciated!
TL;DR: I girl that I liked and seemed to like me started to act increasingly colder after I confessed my feelings for her. It may mor may not be because of stress. She may or may not like my bestfriend. I desperately need help!
Edit: I got a couple of comments saying that I should leave her alone because she basically said no. Well she didn't. Or at least I don't think so. What I failed to mention in the post is that we have talked before and I told her that I am willing to wait if there is a an actual chance of us being together. She said that there is. I also said that she could be honest with me and I wouldn't be mad at all. Been there done that, I still am friends with girls that have rejected me previously. The reason I asked for help in the first place is because I was really (and still am) really confused (even though not as confused due to the help of some great comments and advice). So no, I don't think she straight up said no. I could be wrong though, I admit that.


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