Was expecting elbow/tricep pain when I clicked on this. Is there pain at all?
For now probably select exercises around this if possible while you seek professional help ASAP e.g. physical therapist or physiotherapist.
As for what the issue is you’ve not given much to go on however it is peculiar. Instability, peripheral neuropathy/nerve injury or entrapment, muscle injury and many other things could be the cause.
Pain is usually a pretty straightforward musculoskeletal injury however weakness is more worrying. Weakness is of muscles so your elbow can’t actually be or feel weak. Your triceps and other muscles acting on and around your elbow can be weak.
Are cable tricep extensions weak? Can you press down with your left/affected arm while resisting with your non affected arm? Do you feel/can you feel your triceps contracting or is there no activation? Look up Triceps Reflex and see if it’s present.
That’s just neurological issues however given the host of things that could be going on, many of which we have neither the means to diagnose or treat ourselves, would highly recommend seeing a healthcare professional.