Not eating and I don't know why

Will keep all the same in the tank, and get a heat pad. She does feel cold sometimes. And my apartment temp fluxes because I only use a space heater. We've just had a cold snap here, concerned about this I asked the store keeper abount a under tank heat pad, and she is very against them for burn reasons. I have tended towards wanting one for her due to my situation in heating. I am told she needs the rather warm temps for a warm/cold side.. so when Chocolate would feel cold and stiffish sometimes it seemed so wrong, but the store keeper says "they are cold blooded". (Regardless I started worrying about her getting a cold, thus the concern with her first hissing spell. Hiss or a cold sign) The store keeper has a clean store and healthy babies, but I am thinking little knowledge. Thank You for this site!!!