5.3 is good certainly where I like to be but if not eating and have diarrheal, if over a prolonged period if it was me I would speak to my Dr or NHS helpline if you can get through.
The diarrhoea is a concern, you can become dehydrated quickly even when you are drinking. It is probably worth discussing with your medical support team.
I find I have to reduce my basal insulin when not eating ... again best to discuss with your team ... hope you feel better soon