It sounds like a trapped nerve to me. Do you have numbness in your legs? Or feet? Or foot even? I have a prolapsed disc and kjust today been told I am having surgery to repair it, and I have constant leg pain, sometimes so severe I cannot walk for it. Sometimes I have horrendous back pain with it, sounds like you are having an 'acute' episode. You most definitely need to see your gp for further investigations, as if you valve had this before, it may well be disc related, and I know from experience MRI won't always show things fully at first, it can take a good year or so to manifest itself onto the MRI enough for something wrong to be visible. What did they say when you went into hosp for QES? Something is causing it?
In the mean time, I was given some fab advice for nerve pain from I think it was lequeen (sorry it it was someone else!) who recommended rubbing deep heat direct into the area the pain comes from in the butt, probably about a third down for sciatic pain. Also, heat pad/wheat pad helps me, and funnily so does some gently walking, it hurts to walk, but if I can get myself moving I can sometimes ride it out, not always. Sometimes I take diazepam and go to bed crying.
The red flags are, incontinence, either bladder or bowel, foot droop or numb/tingling saddle area including genitals. But to be honest, and from my own bitter experience of not listening to my pain, if you are in so much pain that you cannot control it at home on your pain meds and they cannot be increased safely, you probably should be in hospital for some better pain releif. I ignored that and suffered, I would say in silence but I moaned on here! Bt didn't go to hospital when I should have. I won't ever do that again, and brace myself for that decision a lot (including now!)