Hi, my 16 year old cat is not too good at the moment

She had some antibiotics for cystitis last week and now she is suffering from runny poo's and is very out of sorts. There was a little amount of blood in one of her poo's but I thought this may be due to her straining? I've held off taking her to emergency vets as yet as didn't like to bother them if it was something trivial and wondered if ne1 had any ideas out there? My wife is really stressed which in turn makes me sad, so any help would be appreciated
