I'd still wager that it's a case of disuse. When your body gets used to doing certain physical activities (ie, weights at the gym) and then you suddenly stop, you're going to feel a bit stiff and sore.
This would explain why the morning is worse -- you go for 8 hours without using your arms, then expect them to work again. As the day goes on, you've been using those muscles doing normal, non-exercise stuff, and it loosens things back up.
If the initial injury has gotten better, it might help to go back to your old gym routine (or maybe slowly working back to that) to recondition your biceps.
If ibuprofin and/or paracetamol help, take them.