If it's hurting when you bend down and/or you are hearing a popping sound, definitely consider a sinus infection or allergies, depending on where you live.
The chance of it being a brain tumor is so small, it is almost unfathomable, so don't let the people who are writing about brain tumors scare you.
At your young ages, 13-18, it could have to do with hormones. The same thing happened to me around those teenage years where I would get crazy uncontrollable headaches. I had to medicate with tylenol or ibuprofen in order to control them, but they eventually went away.
So, don't worry, but get yourself to the doctor for some antibiotics if it is a sinus infection, some allergy meds for allergies, or just sit in a warm bath with a warm washcloth over your eyes. You'll feel better in no time!