I peed out blood? what's wrong with me?

Could be UTI: It could be a number of things depending to your symptoms. Such as urinary tract infection (UTI), kidney or bladder stone, bladder tumor. Since you are 18 years old I would put aside the bladder tumor for a while. You need urinalysis, urine culture, ultrasound of the bladder to start with. Also blood not coming from the vagina needs to be ruled out. Please see your primary care physician.

Hematuria: blood in your urine is something that needs to be evaluated to determine the cause which can range to a simple infection, to stones, to somethings more serious. Get eval.