I was doing pushups today in gym cl, on my 20th push-up, the back of my head suddenly hurt like..?

..someone smashed my head with a brick. The unbearable pain lasted 10 minutes, then it started dying down, but right now I can still feel some sort of pain in the back of my head. Was it a stroke? Was I dehydrated? Was I not breathing correctly, resulting in a lack of oxygen supply to my brain? Or was I just straining myself and going too far? This has also happened to me on the 4th. My cl started weight training on Tuesday and ever since I've been forcing myself to do a lot of repetitions of different exercises. I've also been sleeping at 12:30 midnight the past few weeks. What's the problem?