I just learned today from a previous post that the vagina can produce smegma (don't know why I didn't know this before) and if that smegma gets caught under your clitoral hood it can cause pain. I'm hence going to conclude that after a night of vigorous petting, your wetness might have somewhat solidified and collected underneath your clitoral hood. After your sexual stimulation, make sure you clean the area around your clitoris to keep it from building up. If that's not the cause, then maybe you've overstimulated your clit and the pain comes from that?