GP didn't have a lot to add. Agreed that it sounded like muscle spasm and recommended physio. I have an assessment booked with my physio for next Tuesday. In the meantime I've been sticking to mobility work, light power cleans and some abs. I suspect physio will same the same... muscle spasm, perhaps prescribe some specific work to improve lower back strength. Otherwise I expect the results to be "slow, steady, smart rebuild to peak performance."