Our son had Tommy John two years ago. He felt the problem right away, on a specific pitch. Said it felt like hitting his funny bone, but in his elbow. MRI did not show the tear, but after an attempt at rest and rehab, he had the surgery in which the doctor (Dodgers doc) did a few tests in his office, said it was easy to diagnose and after the surgery said there was extensive damage.
Advice above is good - get to a doctor who knows what he/she is doing on elbow injuries.