Forget drugging your pup. Benadryl is not a solution. Use training. She needs to know that you can't always play fetch.
There are more alternatives. Get a bucket of little balls to put at a comfortable level for you when your back hurts. You can use a broom and dust pan on a pole to pick them up so that you don't need to bend over. Or get an automatic ball launcher:
GoDogGo Automatic Ball Launcher & Tennis Ball Thrower for Dogs - GoDogGo Fetch Machine Official Web Site