Open Question: Can someone explain what is wrong with me? ...or is this just a normal girl / teenager thing?

I'm 16.. and a girl lol. This morning to my mom: "Do I look abnormally pretty today? I feel really pretty right now" that mentality continued until the afternoon when I overheard someone say "yeah she's really nice, but she's ugly" They could have been talking about anyone in the world, but of course I thought it was me lol. So then.. I thought I was the ugliest person in history.. for about 3 hours. NOW.. I just looked in the mirror and I was like "I'm not gorgeous, but I'm not ugly and it doesn't really matter anyway" UGHHHH why can't I just make up my mind? I swear, my day goes like this. I'm ugly then I'm pretty then I'm average. I'm driving myself insane.