I got exactly the same when I used to play badminton regularly then tried to play tennis - I know this sounds obvious, but are you making sure you're hitting it by following through with your arm, rather than controlling the whack with your wrist? I had pain in my arm for weeks afterwards and just after playing dropped a kettle of water because I couldn't bear weight on that arm
Swing Ball is ace.... I have a set but I got so carried aaway I broke 3 balls off it and have had to settle for a plastic ball now. really good therapy if you are stressed. found myself competing with the 9 yr old boy next door!!!