Why is One Shoulder Suddenly Weaker?

I’ve been training for 20 years. The last year I’ve been working towards my long term goal of benching 315 following Westside Barbell type methods.

On my max effort days, I’ve been doing heavy Singles and Doubles.

For some reason, 2 weeks ago my left side has gotten incredible weak.

It’s noticeable weaker in all pressing exercises. My left side even seems weaker in Triceps extensions.
I have not had any pain, so I don’t think it’s an injury.

I can’t figure out what’s going on. Maybe I injured my rotator cuff.

The only thing different I’ve done, is I was working on getting a muscle-up on gymnastic rings.
I did a few where my left arm flared out into a weaker position, but it didn’t seem like I injured myself.

I’m going to take some time off, but I’m just puzzled.
I’ve never experienced this before.

I’m 36.

There is no history of vascular disease in the family.

It seems to be just my left arm and shoulder.

Specifically, I try to keep my elbows tight to my body and rely on my triceps to drive the bar vs allowing the elbow to float out and use the rotator cuff.

I’m having a difficult time doing that over the last 2 weeks.
My left arm seems to want to drift out and my body wants to turn to the right( clockwise ).

My left should just feels unstable, but there has not been any pain.

Not exactly the same thing, I am 37, and recently went in for an MRI because my previously strong right side, is now my weak side. Arthritis was my diagnosis. Even though my shoulder wasn’t hurting in the gym, but rather, later, like when going to sleep in certain positions, etc. I wouldn’t think it would have made a difference if I wasn’t feeling pain in the shoulder and pulling back because of it. I don’t have pain in my right shoulder when pressing at all, but it somehow still made that my weak side.

Kinda different, but maybe not. I would say go get an exam/MRI done, either way.