I was at the gym about 3 days ago and happened to be doing pull ups.. I Felt a sharp pain, almost a pop on the lower left part of the back of my head. This was immediately followed by and intense throbbing headache. I have been to the gym 3 times since this happened and each time as soon as i strain even a little bit, the intense throbbing headache comes back. Does anyone know what this might be? Its important for me to know because i am signed up for the marines and i leave in september so i need to be in shape, but i cannot seem to workout when this headache keeps returning. Thanks for reading.