In reply to JimR:
It may be that you have an underlying issue, which pressure exacerbates. I'm sure Dr Google would furnish you with a number of horrifying 'possibilities' - i.e. an unfiltered mess of fatal conditions - so I'd say a visit to the quack would be in order.
If we can do a sweepstake, I'd go for kidney stones (only joking: actually, I'd go for muscle strain).