Why is my bearded dragon turning black and puffing up?

I bought my bearded dragon as a juvenile from a woman who was selling him due to allergies. He is given a diet of vegetables and the occasional super worm, along with juvenile feed containing crickets and meal worms. Since I have purchased him roughly two weeks ago, I am not handling him often so he can get used to... show more I bought my bearded dragon as a juvenile from a woman who was selling him due to allergies. He is given a diet of vegetables and the occasional super worm, along with juvenile feed containing crickets and meal worms. Since I have purchased him roughly two weeks ago, I am not handling him often so he can get used to me. Since I've bought him, he's been very active. He eats his food regularly, he lounges under the basking lamp and is somewhat social even though he is in a new environment. But for the past 2 days, he has been under his cave more than usual and hasn't eaten his food. His stool is relatively normal, but the last time I cleaned his tank (every two days) I noticed he wasn't digesting his worms well... there was semi-digested worms left.
So, my question to bearded dragon/reptile gurus and experts... what can I change to make him more comfortable?

His current diet consists of zucchini, carrots, yams, broccoli, green peppers and the occasional celery greens. (As advised by beautifuldragons.com)

He was previously in his tank with his cave, one large lounge, his normal lighting system and mats. I have, since then, filled the tank with sand (spherical, edible if digested) and a few more decorations. He enjoys to be misted, so I often to do that. He has a large water dish to bask in, and is given two small meals on my days off, and one large when I'm working.

My dog (sometimes) sits in front of his tank and watches him. I have made sure she doesn't watch him any longer, assuming she may be causing him some level of stress. Any suggestions?
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