There are a few possibilities for what is going on.
Walking in circles is often seen with a condition called Feline vestibular disease this normally causes a head tilt as well
If he has been vomiting today and may not have eaten today then this may be due to a dangerous drop in blood sugar, when a diabetic cat become hypoglycemic they can stumble when walking, loose control of the hind end , become extremely lethargic and even slip into a coma.
Diabetic cats that are not regulated properly can develop a condition called diabetic. This is normall a progressive problem and does not show up all at once.
Dragging the hind legs can be a sign of a saddle thrombus which is a blood clot that forms and travels to the far part of the aorta, in the area between the rear legs.
Read more here
For future I may be best to have your vet teach you how to test the glucose at home.A human glucometer can be used. We suggest having the glucose tested at the vet and then use the machine so you can see how close the two reading are together.
Here is information on how to do that
Because there is a chance this could be due to a drop in blood sugar or a blood clot due to a problem with the heart I would suggest having him seen at your local animal emergency tonight. They can at least test the blood sugar if nothing else.
Please let me know how he is doing,please reply back if you have additional questions or concerns.