Elbow flexibility

I wanted to know how could i increase the flexibility in my elbow. After Im done throwing weighted balls my elbow is nice and loose and I top out at about 89-90 mph in the first simulated inning. Then in the second I can feel the elbow tightening up and im only hitting about 86-87. I can feel that my forearm isnt laying back as much as it does in the first inning. What can I do to have more flexibility in my elbow and shoulder.

Muscles tighten up - not joints (except maybe due to swelling in the surrounding tissue). You probably need to work on stretching the muscles in your arm. You certainly don’t want to stretch out your connective tissue and loosen up the joints as that will reduce your durability and lead to injuries.