CVS has a free glucose test kit (I think you might need a script, but not sure on that). The test strips for blood are about $20 though- cheaper if bought on ebay
It could be blood pressure as well. I have issues with both blood pressure and blood sugar so I always test my blood just in case. Due to the medication I am on my Blood sugar either drops too low, or too high. Stupid side effects! Orthostatic hypotension - Wikipedia, the free encyclopedia
If you have issues with panic attacks and issues with blood pressure/sugar, I would think it would be a good idea to go back to the doctor. Panic attacks can also be related to hormonal disorders so you want to get that checked out.
I have almost fainted on the horse, I also have had issues with getting lightheaded and having my legs collapse from under me when I walk. I have dysautonomia, but it is rare so I doubt you have that.