OK, I know what everyone says about Sterilites and ten-gallon tanks, but...

I've got my adult ball, which I adopted three weeks ago, in a 20-gallon long. Two identical hides, 90 degrees inside warm one (I'm using a Cobra UTH mat with a Herpstat thermostat), about 80 in the rest of the tank, 50-60 percent humidity. Spends his days inside one hide or the other (he switches freely), but he gets VERY active at night, crawling over the fake tree branch made of rubber and nosing at the screen top and otherwise slithering everywhere. He was in shed when I got him, so I didn't try feeding him at first. Wouldn't take the first two times I tried a F/T mouse, but he did tonight with no trouble (what a relief). When I last tried to feed him a few days ago, though (at night when he was on the prowl) he bit me when I reached in, huge surprise because he's otherwise been the picture of docile. Long story short: I'm wondering if the tank isn't perhaps too small. Just seems he's got cabin fever the way he prowls after dark. Or could it be that going three weeks without food (and he was a regular weekly eater before I got him) just got him feisty? Any guesses welcomed.