I'd be inclined to give live yoghurt a go.
I have Long Covid and dodgy guts have been a factor on and off for quite a while so it could just be a lurking symptom too.
Another thing to look at - and not suggesting at all that you have long covid - is the theory around MCAS (Mast Cell Activation Syndrome) / histamine and Long Covid. One of the symptoms is diarrhoea and treatment is anti-histamines, which is probably the last thing you'd think of, but has helped many, including me.