Exertion headaches (maybe) that won't stop

Hi I am 18 years old, just moved into college dorm, and I work out and play basketball regularly. I've never had problems with what I am presuming to be exertion headaches but on my first night on campus while bench pressing heavy weight, I was hit by an intense thunder clap headache in the back of my head that peaked in seconds and didn't fade for several minutes only to fade down to a mild headache/pressure in the back of my head. I took some excedrin, drank some water, ate some food, and got a good nights sleep (which have been known to fix every headache I've ever had before) but I awoke with the same mild headache in the morning. Over the next week I continued to work out and play basketball experiencing more brutal thunder clap headaches and mild off and on headaches while I was just chilling and going to class. I also drank nearly a gallon of water a day and got plenty of sleep. I went to the ER a week after the first major headache and got a CT scan to check for a subarachnoid hemorrhage but it came back negative. The doctor pretty much had no explanation to my irregular headaches and said to take some days off and I did. For the most part I had no headaches just mild pressure in the back of my head while on my break. I returned to campus after a couple days and went to play a pickup game of basketball and got hit by another thunder clap headache on the first possession. So I guess my question is whether or not these are exertion headaches and how in the world am I to treat them??? I've been moderately prone to headaches all my life but some excedrin or a nights sleep would get them to go away every time, but not anymore. In addition, the rapid onset of these headaches are scary because that's never happened to me before either. Any help would be much appreciated. Thanks.