Im 16 and I haven't had my period in 4 months what's wrong I have had sex but not pregnant

Time to see a doctor to have a check up. But there are so many factors that can cause you not to have a period pregnancy is just one of them. Increased activity like sports or long distance running can also change the way your body works on a temporary basis. But it is always best to see a doctor and get checked out that none of the bad things on the list of why you don't have one is eliminated.