Fasting in the winter is normal for older ball pythons. The ones petsmart sells are often hatchlings or just past hatchlings. Mine didn't start to fast until his 2nd winter - he was 700g around then. Younger snakes don't have the reserves to go months without food. I'd HIGHLY recommend getting a temp gun (they are like $15 on amazon) and at least three digital thermometers - one for inside each hide and one for the mid-tank "ambient" temp. Make sure the probes are at snake level - measuring a few inches above the substrate does nothing for knowing what temps the snake is at. Also get a digital hygrometer - it's the only way to be accurate.
My guess is the snake is either too cold (ambient temps too low) or the surface inside the hot hide is too hot - possibly a combination of both. If you are going to go with a cage as small as a 10 gal to start with, you may as well switch to a tub. It's easier to heat / maintain humidity, and darker for less stress. Since your husbandry is already off a little, I'd not try and switch to a bigger tank until you can maintain a smaller one.
A UTH pad will do nothing for ambient temps, but in such a small area adding a heat lamp / CHE could easily over-heat the tank. You will have to start with a low wattage bulb - CHE is what I'd recommend but you will have to work to keep humidity up in a tank with aspen bedding - and this heat source would have to have its own thermostat or at least be on an adjustable dimmer. You'll want the ambient tank temps to be above 77*F and below 82* (other members may suggest something a little different)
No cage is too large - nature is the best template - a snoot can't be booped too much
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