What is the temp directly under the basking light? Check it, as another possibility is a thermal burn based on where they spots are (spine, top of ribs, knee). The season is getting cooler and days shorter...maybe its spending more time under the basking light than usual. Burns result from high temps but also from too much TIME exposed to that heat level. If you go to a vet ask them to use a diagnostic black light to scan for fungal infections...many fluoresce under black light. Possibly a skin biopsy to tell much.