Pec strain or damage

Can't rush these things. If you do, it will only take much longer to get better. At one time, many years ago, I boxed amateur and several times during sparring I'd have my left shoulder separate. Come to find out, it was a congenital issue and prone to do that.

On more than one occasion, I'd try to get back into the gym before my shoulder was better and all that did was make it easier to re-injure, and it usually did.

Sixteen days out isn't a long time. Work on your cardio with running to keep that up.