Feelings constantly changing

So basically about 5 months ago I made a new friend who had just moved to our school. At the start of the friendship it was great, she even brought me and another person closer together sparking a new friendship.

But as the friendship went on I found myself not wanting to go anywhere with them and kind of hating them. I was kind of worried because of how good of friends we were but I just carried on hanging out with them. Then I began to really like them again which was strange because not so long ago I really didn't. This was weird because I have always had friends and this had never happened before.

Then a couple of months later I got a girlfriend and I really liked her and it was all good. But then maybe a week later I began to think that I really didn't want to go out with her anymore and that I didn't like her. But the same thing happened again and I did like her later. This keeps happening where I go through phases that I don't like them but then I really do. Anyone know what's happening?