Sudden sharp pain: any hypotheses?

I know, I know, I always tell others to see your doctor—but I could at least use a working hypothesis. Seeing my doctor is much more of a challenge than it used to be. It would help if I could do something for myself, and also if I could classify this condition to narrow it down a little bit.

Sharp pain. Focused in one specific spot.
Onset: yesterday. Increased today, steadily, over several hours. Leveling off but still very sharp.
Location: about 6” beneath left armpit, measured from the uppermost recession of the armpit, and a couple of inches toward the rear from that point. There’s a rib there, but no reason to think I’ve injured it.
Relieved by direct pressure from my left hand. Relief only while pressure is applied; resumes with no change when pressure stops. Because of location, keeping pressure on is really awkward, causing fatigue of the hand and strain on the shoulder.
No felt lump or abnormality at the site.
Nothing seen by close visual examination or palpation by my husband and no apparent difference from the other (unaffected) side.
Nothing that I’m aware of has happened: no injury, unaccustomed movement, unusual stretch or impact. No identified correlation with anything.
Never had this before, and it doesn’t feel quite like anything else I have ever had before, which would include muscle strains, fractures, and dislocations in various places.

Ideas? Thank you.