Any news on this?
I was thinking of blocked anal glands, too, but if so, the vet needs to look at it, or they will burst.
My rainbow cat Steffie used to have this, she never showed any pain or discomfort, unless you touched it, so the first few times we did not see it till she had a horrible open wound, because the thing had burst. There is nothing a vet can do at that stage, just give AB in case the wound got infected.
After that, we used to check on a recgular basis, and as soon as she seemed to develop a bump, we'd take her to the vet.