Maybe just a bump in the road (a bad day)?
Absolutely could be just a bump in the road. Getting a cat regulated is often three steps forward and one step back. Giving insulin is never an exact science, because you can never tell what the pancreas is going to do. One day, it might decide to make some insulin on it's own, and the next, it might decide it's tired and take a break.
Testing and knowing the numbers helps, but even the numbers can be confusing as they go up and down, some days seemingly for no good reason. Be patient, and know that your good care will help Thomas get well in the long run. It just takes time.
Other folks have given you good meter and food advice. I just want to agree that you don't need the expensive prescription food to help Thomas. My vet's office sent us home with a bag of dry Hill's DM food. I took one look at the ingredients and refused to feed Billy that filler-filled food. Like you, I don't feed my cats crappy food. Unfortunately for my Billy, while I usually fed him Orijen dry (too high carb for a diabetic cat, but lower carb than most drys) he had access to my mom's cat's food, which was grain free but still high carb. Then he had to have a steroid shot, and his lurking diabetes suddenly got much worse.
We ended up feeding him Fancy Feast Pate. I used to think this was the McDonalds of pet food, but was pleasantly surprised to find it's got better nutrition than the expensive dry I used to feed. It's also simply healthier for a cat to eat wet food than dry. They are notorious for not drinking enough, which causes all sorts of health issues for a cat that mostly eats kibble. Do look at the food list other members posted. It's absolutely true that you don't have to buy expensive food to help a diabetic cat. Billy went into remission eating nothing but Fancy Feast, and he is felling better these days than he has in over a year.
And use mmol/l. The 
She continued to use the human meter too.
