this is actually normal cat behavior. You're just not accustomed to seeing it.
I have to say, if you are giving insulin, you need to be home testing. Can't stress it enough.
Some vets will "suggest" this, but most won't even mention it. They will send you home with insulin and an amount to shoot and maybe some instructions about hypoglycemia (blood sugar dropping to a dangerously low level).
Well, the thing is, human diabetics don't EVER give themselves insulin without checking there BG to make sure it is safe to do so, so why shouldn't it be the same for our kitties. Here on FDMB it is. You will notice that the vast majority of people here test their cat's BG at least 2x/day (before giving each shot to make sure the level is safe enough) and periodically at other times to see how the cat is responding to the current dose. We use a human glucometer, test strips and lancets - which are all very readily available and easy to use.
Our kitties get lots of love and treats for "putting up" with this and most of them actually come out to be tested on their own 'cause they want those treats . Here is a collection of great links that "Carolyn and Spot" pulled together about hometesting. See what you think ... it truly is the best way to not only keep Your cat safe but also really get a handle on this disease and help him to live a healthy life with FD (feline diabetes).
I am giving insulin, but as an easily stressed epileptic (stress can make siezures more likely for me) I am having a worse time with it than Marmalade is. I will start testing when I am more used to "shooting" him. I do understand your reason to stress it though. My vet did not mention testing, but did give me paperwork about it and hypoglycemia, and I plan to ask her about it when I speak to her later this evening. I will need to get used to dealing with a diabetic cat before I will feel comfortable determining doses by myself etc. I have printed several articles from here, and part of tha paperwork from the vet are attached to the refrigerator. I know that human diabetes testing materials are available because my genealogy is loaded (I mean alot) with diabetics of both types, the first one I have as recorded was my moms great granpa, and he (as a very old man) had his legs amputated back in the 40's because medical treatment was limited then. We already have lancets etc in the house because both of my parents are diabetics, but do not use insulin.
I do want him to have a long healthy life, but I still have to work with my vet and my neurologist, so we both have to make compromises.