When T was first diagnosed & I got Cornell involved & we discussed using Lantus, the vet told me the recommendation for using the large vials is 5-6 months max if refrigerated & stored properly. Otherwise, efficacy starts to become an issue.
I never used it because Trupanion covered the Lantus so I cannot share any personal experience, only what was recommended if I had gone that route.
While the insulin can be an issue, the hard belly & irritability also sound concerning

. Constipation or even lack of motility can be an issue & cause increased BG even without there being straining. In fact, T had the issue one time last Fall & his BG went very high because of it. I ended up getting an X-ray & they decided to do an enema in the office. A small, but very dense hair filled piece of BM came out in the process. He was a new man after that with lower BGs. I am not saying that this is Butters issue but it always something to keep in mind/watch for.
I am going to attach some poop charts

because who doesn’t need a poop chart to start their Wednesday

? I am attaching 2, Bristol and then one from Purina. Bristol actually used Purinas for their most recent study of the feline Microbiome. It is a nice chart of actual BM vs drawings

. Also, here is a link on Neuromodulation for Constipation:
https://www.veterinarypracticenews.com/neuromodulation-for-constipation/
It has some good info & explanation on simple things like massage of the belly (T loves this...not if having an issue...more so when he is doing well & I do it to help him out), probiotics & how they help from a neuro perspective. Probiotics may be helpful as well at some point

but like everything else,
one thing at a time when possible so as not to confuse things. If you do decide to use one, start slow & increase with time to make sure there isn’t any gas etc..
The only other thought is just to keep in mind that in a high percentage of cats diagnosed with “just” pancreatitis, there is actually also intestinal &/or liver involvement....especially intestinal involvement, so working on that can be very helpful.
Finding what works best with diet, probiotic is simply usually trials & time. In some way, it is good because we know that felines with DM, even if they go into remission (
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7242727/) continue to have an abnormal gut so you will actually be addressing multiple issues at once by working on it

.
In the meantime, I always get in touch with my team when something seems to be rather uncharacteristic or off in an attempt to be proactive. Essentially I call a “heads up”. If it turns out it is something simple, no big deal. If it turns out I need them, they are on the same page & ready to help

with next steps to figure out what is going on.
I hope Butters feels better today

.