Regarding upper back issues of mine, overused and stretched muscles, traps / rhomboids etc. what causes the muscles to give a burning sensation ?

Soreness: The timing is important. There is a myth that it is caused by lactic acid, but this is usually cleared within a few hours. If it is delayed onset, it may be from "over-repair" of damaged muscle fibers. If it is persistent it should be worked up.

Exercise or strain: New exercise or activity? May cause dom delayed onset muscle soreness (typically about 48 hours after the increase use of those muscles). May last a few days to a week or two. Alternatively, sudden/acute burning may indicate a strain, which may take a few weeks to several months to resolve. Massage or foam roller may help, if this is not a muscle strain.