For a natty, you really need to be hitting each muscle more than once a week. If your muscles need a week to recover, you're doing something wrong. The 5 day bro split leaves too long of a wait.
I've had this many times...tendonitis. You'll need to take some time off from lifting anything with that arm. Ice it in the morning and evening for 20 minutes and just wait it out.
All in your head, with DB's you're most likely trying harder subconsciously since there is a higher risk factor with free-er weights. If you can psych yourself up the same way during BB, you can...