Thanks for that tip Donette - having had a vile night. Both SIs playing up, left side rib cage then...dada...pain, flush of pain 'from' right SI down back of leg and into...achilles heel. Lubly-jubly. Just taken a pain med as am out driving pony this morning. Going to have a hot shower, when the bathroom has heated up some (no C/H in the house, electric rads).
There ya go Pianoplayer - much like as you reported! Mine is definitely SIs - feel the two points burning away, then the flush of pain (about a 3) down the back of the leg, to the achilles in right ankle (about a 4). But *no pain on top of foot. Waking to get going this mornng was diff, climbing the stairs was...well, did it. Not without some grunting!
What a vile night. But will pass. 'Smile'.
