6yr old ds2 awake in middle of night with pain behind knee - in tears

[lborgia]

It's malt 2 am here, ds woke up earlier with pain in the back of both knees - so bad he can't move his legs,which are bent 45 degrees and seized up. For the last 2 nights is been just the left leg and only woken up at 7 am Ish with it, but now both knees and middle of the night. It's taken several hours of limping both mornings until walking normally.

Gp didn't know but said nothing to do with weird rash/virus/exhaustion of last 2 weeks
He thought cramp?

Actually feel better now it's bilateral, but behind knees is a new one for me.

Any ideas/experience?

Bump?

What sort of rash? Have a google of henoch schonlein purpura - DD3 have this last year and had quite severe joint pain in knees and ankles and less so elbows. Rash on lower body and slight on arms. For her no kidney involvement but can happen with some cases.