I’ve had episodes of severe backache with MS, but have never quite got to the bottom of whether they’re a relapse or not, because I don’t (usually) get neuropathic pain or other nerve symptoms with mine. It’s just boring old painful pain, that’s more likely to be caused indirectly, by poor gait or posture.
But, since you have symptoms that feel like a burn, I’d say there is nerve stuff going on in your case, so sounds more like a relapse than an injury.
Might be worth speaking to the doctor, just to get it recorded it happened, but I doubt they’ll do very much, unless it’s severe and you want to try steroids!
If you’re not diagnosed yet, it all adds to the evidence (even if they don’t treat it).
Hope this helps,