Do I have a cavity or are my teeth just moving around or sensitive?

I am 16 and when I was a baby I had antibiotics which caused my back teeth to become mottled slightly on the sides. The last 2 days I've had a strange feeling in my back 2 teeth it was painful it just felt sore and throbbed without hurting at all, I've eaten on it fine for the few days and hasn't been sensitive to anything hot or cold. I had a look in the mirror today and my back tooth looks a bit more mottled and I think may be slightly indented now but didn't hurt when I touched it. I can't tell which tooth hurts but could the mottle have turned into a cavity?:( I showed my mom and she thought it was just a mark I hate the dentist so would be last resort for me to go there! It feels like it isn't that tooth that hurts though the feeling alternates between my back 3/4 teeth?