Tortoises and heat mats do no mix, at all. I would advise you remove this a.s.a.p. Tortoises need to be able to 'see' their heat, so unlike most other reptiles, leaving mats underneath them can be dangerous.
It is possible it is trying to escape this excess heat, so I would remove and see what happens. Can I ask what your highest/lowest temps are? Do you have an outside enclosure for your tortoise?
You should come over to the shelled section, we are nice (mostly