What Causes Swelling In The Testicles?

Hello, my testicles are swollen in certain areas. Its a reaction type of swollen. It burns really bad when water touches them and they are very sensitive to the touch. Last night my girl friend did not feel like having sex so she jacked me off using lotion. She has done that before so its nothing new and its the same lotion she has used before also. I'm positive that its not an std. Its some kind of reaction but I don't know why and don't know how to help the swelling and mostly the sensitivity!! It hurts pretty bad when touched. Can someone help? Thanks