Short Answer 
Cremation involves exposing the body to temperatures between 1400 and 1800 degrees Fahrenheit, which rapidly consumes all organic tissue, leaving only inorganic materials. Since the nervous system is destroyed in this process, the body cannot experience sensations or pain, making the idea of feeling a burning sensation impossible.
Cremation Temperatures
The process of cremation involves exposing the body to extremely high temperatures, typically ranging from 1400 to 1800 degrees Fahrenheit (760 to 982 degrees Celsius). At these intense heat levels, all organic matter, including body tissues, is rapidly consumed. This process leaves only inorganic materials such as bones and metals behind, which means there’s no intact structure to experience any sensations.
Destruction of Organic Tissue
During cremation, the belly button and other organic tissues are among the first parts of the body to suffer because they are composed of soft tissue. As the heat increases, these tissues decompose quickly and cannot remain intact. Consequently, the idea of a burning sensation from the belly button is invalid since it would not survive the extreme heat of the process.
Lack of Sensation
To feel sensations like burning, a living and functional nervous system is required. Once the body is subjected to cremation temperatures, the nervous system ceases to function. Therefore, there is no possibility for the body to perceive pain, and the concept of feeling a burn in the belly button during cremation is physiologically impossible.