I don’t mind you asking, but I’m not sure I have any nice, definite answers for you.
MS pain can be very complex, and from a variety of causes.
“Stiffness” sounds like spasticity, as I think that’s exactly how most people would describe it in everyday life. Even those of us who know the medical name for what we have is: “spasticity” would probably describe the feeling as “being stiff” - but particularly the muscles, not joints.
Spasticity can itself cause pain, because the muscles don’t relax properly, even when you’re asleep, so it’s fairly predictable that they don’t like this very much, and start to hurt. For people who have spasticity, it’s usually a chronic problem, but that doesn’t mean it never varies at all. It may respond to gentle stretching, or a hot bath (though some people with MS can’t have a hot bath because they’re intolerant of heat), or, if not yet too ingrained, just from keeping on-the-move.
Burning pains, however, don’t sound like spasticity. This sounds more likely to be nerve pain - also very common with MS. Nerve damage can cause signals to get garbled, which the brain may then interpret as pain. It’s a sort of built-in safety mechanism, because if the brain doesn’t understand something part of the body is trying to tell it, the safest thing is to tell you it was pain - the standard signal something’s wrong. Unfortunately, if it was just a garbled signal, and NOT a cry for help from part of the body, this results in uninjured things still hurting.
Finally, you can have musculo-skeletal aches and pains that are nothing to do with spasticity or nerve pain. MS can cause poor balance, altered gait, and weak core muscles, among other things. So imagine what happens if you’re permanently “walking funny”, or “sitting funny”. Eventually you tend to get aches and pains, from muscles and joints having to do what they were not designed for.
Even for diagnosed people, it’s not always easy for them or their doctors to work out which pain is caused by what. I think my pain is mostly caused by spasticity, but I’ve been treated for nerve pain, “just in case” - which had no effect,so we (my neuro and I) concluded it couldn’t be. But sometimes the best (only) way to prove the type of pain is which drugs it responds to. If anti-spasticity drugs don’t help, then the pain probably isn’t linked to spasticity, and so on. I know this all sounds a bit arse-about-face; usually you’d determine the cause before trying to treat it, but with MS, it’s not always like that. Sometimes it’s try to treat it, and find out what works - then you know which type of pain it must be.
Sorry if it all sounds horribly hit-and-miss. With MS it’s often a case of: “Try this, and see if it does anything!” If you’re lucky, it improves something right away. If you’re unlucky, it might be a lot of to-ing and fro-ing, trying different things, before you find something that works.
The good news is that there’s very rarely only one thing that can be tried, so if you get no success with one type of pain relief, it’s always worth trying another. And also you should not need a confirmed diagnosis to get pain relief.
I’d never heard of Zapain - are you in the U.S? I think here, we call it Cocodamol. Won’t work for nerve pain, but might help with spasticity - but only to mask the pain, not to address the underlying muscle tightness.