Hopefully someone who knows more about antibiotics and gut health will see this thread and advise - but from what I've read, it seems like antibiotics can destroy gut bacteria which could then cause those symptoms. I think it's do do with antibiotics killing all the bacteria, both good and bad.
You could definitely try probiotics and other things to help with gut health like Dynamito said, I don't think any harm would be done by trying that. When I've had dodgy bowels before I've used probiotics and it's taken about 7-10 days to notice a difference. It might not be hugely helpful depending on the issue, but it shouldn't really make it worse.
Good luck with the appt! Try and take what they say as either reassurance, or if they want to do tests, that they're just covering all possibilities no matter how unlikely they are. It doesn't mean they think you are seriously ill.