I use Cobra brand heat mats for my leos with no problems. They do not have an adhesive, rather, they are sealed units that can be attached with packing tape and removed later with care. They can also be used inside of the tanks. I have used the same ones for 2 years now 24/7 and they are still operating perfectly. I highly recommend them. The can buy them for a reasonable price through herpsupplies.com.
I also wanted to comment on two things I am reading here in your replies. First, avoid human heating pads. I used one once and it developed a hot spot and began melting. If I did not catch it when I did it would have started a fire. It was a sunbeam namebrand so it can happen to any one. They ARE NOT designed to be on 24/7 I also know many people use lights, but I used lights when I first had my leos and they were less active, and had many more problems with shedding and eating. Since going to the cobra mats - I have had zero instances of eating or shedding problems, and no malfunctions of my mats. Just my .02
Finally - I see you think it may be the mat. I do not know how your vet operates, but mine allows me to just have the fecal done for 15 bucks. I would do that if you can with yours, just to be sure. Good luck.
-Sean
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livXpreshun 
1.1 High Yellow Leopard Geckos "Halo & Storm" [seperate cages]
0.1 Patternless Female Leopard Gecko "Wave"
"It takes a certain maturity of mind to accept that nature works as steadily in rust as in rose petals." - Esther Warner Dendel