Since I misposted here, I might as well use the spot to say first shot two days ago. Arm was sore day after, oddly my other shoulder was sorer than usual, but that's usually the case if I haven't been throwing a ball around enough (mildly blew it out throwing too hard when I was young or something, gets better if I throw a ball frequently such as playing ball) so I don't think that was due to the vaccinne. I slept most of the day yesterday and was kind of a grrump when I wasn't sleeping, woke up with arm feeling better and not feeling unusual.
Not vaccine related: I had a very bad case of neck and shoulders stiffness and pain earlier some months ago, had to lay in bed for a few days before I could sit up due to neck pain (bathroom was only place I sat down for three days, agony in my neck to do so). Then I had to carry my arm above my shoulder for a few weeks afterwards or my shoulder ache'd progressively worse whereas if it began to ache it progressively lessened with my arm in place above/behind my head. Not sure if I had covid at some point, I was pretty careful when I did go out, but maybe, since I was handling out of state/country produce shipments for my job in the months before the stiffness; if I did it would have been months before the stiffness I think unless I was completely asymptomatic except for stiffness.
Initially it started in my (non throwing) shoulder, but the next day I woke up and my neck couldn't support my head's weight without a lot of pain. I thought I had just slept on it wrong, but it persisted for longer than one would think for that. After my neck was getting better, it was my shoulders, though it was worse in my non-throwing arm and that is the arm I had to fold over/behind my head for a few weeks until not doing so did not cause a progressively worsening ache. The arm was fully useable, though I appeared to lose a deal of muscle from it, because I had a spaghetti arm by the time my shoulder was better. It was quite odd, never had that happen before.
On the bright side, it seems to be completely better now. Also another possible and maybe more likely reason for the injury was that I had been attempting handstand pushups for the first time in a while at some point close to the stiffness beginning, but I haven't had that problem before when I did those (generally when I don't have a fluffy big American belly like I do sometimes; I don't want to get slapped in the face during a handstand)