Mali uromastyx sudden death?

I got a pair of mali uromastyx about a year ago, rescued them from bad conditions. They both were active and looked healthy, they ate great and made a huge mess for me everyday. I noticed that one had a lump on his face under his eye, so I took them both to a reptile specific vet and he told me they looked great and I was doing everything correctly. He said the lump was probably an old wound and would fall off as he sheds. So I didn't worry about it too much. Then, a few days ago my perfectly active sweet baby uro was staring blankly at me when I got home. I took him out of the tank and he was limp, like his tail and back legs but his front was stiff. He kept opening his mouth and shaking his head it looked like he weizuring... I was horrified and tried to take him to an emergency vet at 10:00 at night. Since the vet had to know reptiles I had to drive 30 mins away. He died on the way there. I am so angry that the vet didn't look better at him. I know how to take care of reptiles.. So I don't know why he died. I want answers. I am just wondering what could have killed him. Do you have anu ideas? I'm kicking myself for not trying another vets opinion after the first told me he was okay. I feel bad that my other uro is all alone now too! :( thanks for your help.