Chams do not twitch because of their bones. dont know wher you heard that.
muscular twitching is a sign of MBD.
Muscles require a certain level of calcium to function properly. Without it they twitch. The twitching isn't because of the bones. It is a symptom. Slightly off topic but just by way of passing on information, egg laying requires calcium for the muscles involved- sometimes egg binding can occur if calcium levels in the muscles are too low as well- the mother won't even twitch and her bones may look good.
That said- the twitching is pretty unique. It isn't like a quick "chills" spasm or something.
Here it is in a bearded dragon-
https://www.youtube.com/watch?v=keUmGN6BhPk
90 seems a little hot. Only thing I can pick out.
Your basking spot for a cham that young is too high, try to lower that to like 85 at the highest.
As someone who has been raising baby chams at the same temps as adults for 22 years now- the twitching isn't from the temperature. Whatever it's cause it isn't too warm a basking spot. Lower temps for babies basically have 3 reasons- the babies can dehydrate faster if not properly kept hydrated, they heat up faster and can cook if there is not a proper thermogradient, and lowering the temperature slows the metabolism and the growth rate. Slowing growth rate can help prevent or slow MBD from improper diet/supplementation. Because slower growth= less demand for nutrition. It has nothing to do with what the lizards need/require in nature- it's more like a safety net that slows problems from errors in husbandry.
ORIGINAL POSTER-
What is your brand of UVB lighting, and how far is it away from where your chameleon spends most of its time? Was the bulb purchased at the same time as your chameleon?
A little uncomfortable telling you what to do, but if this were my lizard I'd probably replace the UVB tube, make sure the lizard can approach it within several inches, and temporarily up the frequency of the calcium with d3 and multivitamin for a couple of weeks to see if the problem goes away.
Or maybe if I was less experienced I'd go straight to a vet.
Keep in mind though that the twitching from MBD is long and drawn out. A fast spasm like a cold chill every once in a long while (few days between) is probably nothing...