My elbow pops when I do push-ups or any exercise that requires me to bend my arm 80 degrees or more. What could this be? (only my left elbow)

See below. Problems like these can only be correctly handled by your doctor in person. He/she needs to listen to you, perform an examination and possibly run labs or other tests. That's the only way he/she can find out what's going on and what to do about it.