Wrist Starts To Throb. Why Does That Happen? Can I Prevent Injury During Push Ups?

I work out every other day or so. I usually do three sets of exercises. Most of the time, each set includes 30 pushups. But something recently started happening that prevents this. By the end of the second set, my left wrist starts throbbing when I move downward--the more pushups I do once this starts, the more intense the throbbing becomes until it s so distracting and worrying that I have to stop. It s literally painless, but I can t help thinking that if I were to continue while feeling those sharp, jolting throbs, something bad could happen. What could this be/how can I prevent injury and still include pushups?