It is good the pad early feels barely warm extremely crudely that is close to correct. 90ºF is below your body temp. You should trust a thermometer not your hand!! No a burn would be red, and the pad would feel quite warm. modern pads rarely get hot enough to burn these days, buy a new low density unit rather than a high density one. Ultratherm, zoo med are my favs.

Without a photo it is hard to say much of anything. Belly scale striations happen much like the ridges your finger nails get. There is no darkening of color (not pigment spots) around the rough areas? I would guess this is scuffs in the scales themselves. They are essentially the same as human fingernails (both carotene) so they mark the same way.

I would suggest two things a very close inspection for a sharp place where the damage could happen. Check the enclosure and all the items inside as well hides bowls etc.

I would correct your humidity issue. The first thing would be a larger water bowl, larger meaning a shallow large area not deeper. This will increase the water vapour. Depending on the enclosure you could alter the way the air flows as well. Never block off all the venting! A simple change will work. Sometimes obstructing vents located high on the sides helps a lot. Picture blocking half of each of the high vents. If you have a fully open top (like a scree top tank) I would block the middle third leaving openings on either ends. Placing the water bowl on the cool side can make a difference as well. Substrates can make a difference as well, coco coir or cyprus hod more than most, papers reduce humidity.

Without images this is a best guess. Nothing more, from the information you have given it sounds like scratching or some sort of abrasion to me. Try to get a photo there could be a number of other quite serious issues.

Is there any foul smell at the areas? (get close and sniff, old or going bad smell)