It's an exertion headache. It occurs during heavy exertion, usually a sharp pain in the back of the head/neck that leaves a lasting pain from several hours to a day or more. It can occur from not breathing properly, over exerting, and even during exertion while having sex. It will subside over time, but once you get one it will tend to come back more easily (usually for a few weeks after you get a severe case). After a while you should be able to feel when the headache is about to come. The pain is usually delayed somewhat and you can feel it build up during exertion so you will know when to back off. I'd suggest you take some painkillers and wait till the pain is completely gone before lifting again. Lighten up a little bit and be careful when you do any strenous activities.