This sounds like what my daughter has complained of in one or two of her past seasons. It's likely it is tendonitis and you have some inflammation going on. Try resting it, icint it and some anti-inflammatory meds.
Ah ha. I believe this is Pitcher's Elbow, a form of tendinitis caused by overuse. Tendinitis means you have micro tares in the tendon that occur over a period of time and these tears have not been allowed to completely heal because of continued activity. For softball pitchers, the tendinitis is usually on the inside of the elbow rather than on the outside,. This might be what you are experiencing. If so, I would advice seeing a doctor and in the meantime...
- limit serious activity with this elbow (especially pitching)
- ice often
- take anti-inflammatory (ibuprofen)
- brace the injured elbow