My first really big flares were exactly like that. Really painful SI joints, and incredible muscle spasm through pretty much the whole buttock area. It did make it absolutely impossible to voluntarily shift my legs forward. The only way I could do any movement at all was with two crutches.
You really want to knock the inflammation to stop this happening, but you might need to have a back up plan. I always have a bottle of diazepam available, and that makes a pretty good short term muscle relaxant. Also, you might want to get yourself a pair of crutches to be able to at least use around the house when things are really bad.
I don't know about how common this major kind of SI muscle spasm thing is, but I know it wasn't recognised in me as coming from SI at all - I ended up in a neurology ward, and when they couldn't find anything neuro wrong they put it all down to me being a hypochondriac and just putting it on. Its only five years later (and five years of walking with crutches) that it is becoming clear to doctors that I have something inflammatory going on.
