In response to all the people who replied to me yesterday, thanks for the concern and support. 
I didn't get an ambulance or anything, eventually the pain sent me to sleep and I woke up fine this morning. Thank god. I've been alright the rest of today but I never left the bed, but now the pain's kicking in again. 
Apart from feeling moderate to severe pain I'm feeling dread, I have physio tomorrow and I believe physio was the cause of my pain in the first place. They were really rushing me and saw me way too soon after my op and they took my splint off me and instead put a very painful device on that provides no support and presses and rubs against my wounds. They'll probably try to have me running tomorrow, though I've been bedbound and hardly able to think since the op. Oh well.
Don't they ask for feedback on the last session, to see if they went too fast or to slow? That is common practice here at the beginning of a therapy session, briefly ask how things went after the last time. And they will adapt the plan of "attack" if need be.
Hope they can tell you what it was, and that they can prevent a reaction like this happening again.
ponds from now on.