Lou suffers from this every now and then.
COULD be a pinched nerve, key is if steroid calms it down.
Lou gets Dexamethasone when needed.
I never know when it will happen so I always keep them on hand.
For now, take to vet or e-vet ASAP so she gets out of pain.
Rest, no long walks and no up and down stairs.
Good luck and keeps us posted.
Your girl should have a harness as opposed to her collar, might be tough to put one on her now while inflamed, but do think about it.
Lou wears his collar for ID and harness for activity.