Lower arm slightly numb

Sorry we can’t diagnose you and I suggest you see your dr. What you are describing might turn out to be carpal tunnel syndrome or something else unrelated to your diabetes.

I get that sometimes if I sleep with my weight on my arm. If it persists then refer to @Rachox 's post.

Yes. see a doctor if you get it again. I got something similar while walking a couple of miles home from town center. It stopped very quickly after I sat down. So I went to the GP and she said ' that could be a cardio symptom'. Had tests and confirmed I had a cardio problem.
Not so likely to be that in your case since you weren't doing any exercise, but I had previously though that Angina was always a pain in the arm - not a numbness or a tingling sensation etc.