Ok have a question about my kitty.....(a lil long, just so u kno)?

My British Shorthair Misty is a bit overweight, she was like this when we got her & tho Ive been trying to slim her down, she is a sneeky one. Anyways, I just needed to note that she is a bit portly before I went on....despite her buddah like belly(add to that a spay belly) as of latley she seems to look a bit rounder. Also her poo had been smellier than normal & upon further examination I found some poo stuck to the hairs by her bottom. I cleaned her bottom & gave her a sanitary clip to help prevent this. ( I know sometimes Garfiled-esk kitties can have a hard time cleaning thoroughly)...well while cleaning her butt with a wet wash rag I was applying presure & some puss came out. Not a large amount but puss is puss none the less & indicates infection. It does not appear to come out of the anus but more like an abrasion or sore just inside. SHE HAS A VET APOINTMENT this Tuesday & other than the fact there is puss in no way seems to be uncomfrotable or ill. She is eating, drinking, & pottying normally...her actions are normal & if not for the more potent smelling poo that led me to think to check her bum I wouldnt have noticed her different in any way. Im just wondering if anyone has experienced anything like this & can maybe give me an idea as to what Im looking at......its been suggested an infected anal gland or possible blockage. As I said shes going to the vet Im just concerned & curious if anyone has experienced anything like this in their own kitties.....Thanks♥