Drink more water, eat a banana, lol potassium, maybe stretch a lil. Try doing bench first, maybe your getting such a pump in the quad that its just ready to lock up and when you start flexing it to stabilize your bench press, it gets pissed and goes on lockdown. Has happened to me plenty of times, usually thou, its a damn spot in the middle of by back, or right under my shoulders... Hard to focus but ya gotta work through it.