There is no visual swelling and nothing looks wrong but I'm having intense pain in my middle finger and can't even hold a baseball, what happened?

Need more info... 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.
A couple of things. It could be early arthritis of your finger joint, or, tendinitis. Both are very common. An x-ray could determine if there is arthritis. An orthopedic evaluation could determine if it is tendinitis.