I'm depressed but I have nothing to be depressed about. Help?

So just a little bit of background I'm 15 and I'm generally an extremely happy person and I'm always smiling and everyone knows me as the kid that loves life and all that. I even act as a sort of councilor to some of my peers at school and try to make them feel better about themselves. I am their vent.

Recently, however, I have been feeling extremely out of character. I catch myself sulking around the house, and feeling extremely tired and weighted. I have never felt like this but from what I have read this is what depression feels like. I have a hard time believing that I suddenly became depressed but there are a couple things in my life that haven't quite gone as well as I would like, but I mean the way I have been acting it is getting a little ridiculous. Even my mom has noticed the sulking, and so I lied to her and told her I felt sick, because I already know what she would say. She says the same thing every time. To just think through things logically. I HAVE! This is why it's so strange to me. I have already thought through everything that is going on, and logically I shouldn't feel so crappy.

The only things that I have in my life that aren't going so well are

1: My mom just got remarried to a guy that I really don't like

2: My girlfriend has been acting strange (more detail below)

3: I'm lonely (maybe? I don't know)

My mom getting remarried is pretty straightforward. She got married to a guy that I don't like. He is like 15 years older than my mom and likes to debate about everything. My girlfriend is a longer story. I met her in driving class a little over a month and a half ago, and dated for about a month, and then she texted me out of the blue and told me she wasn't ready for a relationship right now. We talked it out and we decided to stay together but I still am not sure if we are really together. I really like her and I think that I love her even tho it's only been a month. It might not be true love but it is certainly not lust. I have been under the influence of lust and this is different. I may not love her but I care about her a whole lot but all she is seeming to do is distance herself from me. We were supposed to go see Evil Dead together this weekend but now she says she doesn't want to see it, yet she posted on Facebook that she still wants to see it! I'm confused. We live 20 minutes from each other, and because I can't drive yet, we don't see each other a whole lot but we text/snapchat/call on the phone every day. I asked her what is wrong and she said she was pissed at the world. She has some serious problems in her family life and at school but she would never act this way towards me unless I did something wrong. WHAT DID I DO. I just want her back shes so perfect and beautiful and she is acting like it's over and it's making me sad, but we only dated for less than a month and I have broken up with girls that I have dated for longer so why do I feel so terrible about it this time? Are all girls this confusing? Is this what life really is like? At this point I am pretty sure that it's over and I have been trying to distance myself too but It's hard because she is SO perfect.

Please someone tell me why I feel like this. I am only 15 so I don't have experience in this world yet and I need someone with more experience to tell me that I am being stupid about all of this and tell me why I am sulking. I think it is because of my (ex)girlfriend but we only dated for a month and I only knew her for a month and a half. I shouldn't be this upset! Why am I upset?! Any responses would be much appreciated I just really want some help. I have no one to talk about this with so I need some help from you guys.