I'm a pitcher in college and my middle finger has started causing me so much pain I can't even hold a fastball, what's going on?

Many things... If the pain is severe and limiting or affecting your activities you should definitely seek medical evaluation. You could have a variety of problems including a stress fracture, a sprain, a traumatic fracture, a trigger finger, tendinitis, nerve injury, or many others. Additional details, exam and possibly imaging would be necessary to determine the nature of the problem and best treatment.
Finger pain. Probably a flexor tendon inflammation. Commonly seen with repetitive finger flexion (gripping a 4-seam fastball, etc). Start with 7 days of an otc anti-inflammatory medication.
Possible tendinitis. It could be tendinitits or even arthritis of the middle finger joints. It could even be the beginning of a skin blister. Have your athletic trainer or team physician check it out.