Anyone recall a B-class "thriller" where a blob like monster lives in the sewer and suck people into their toliet seats? and in the finale the monster shows up in the city and it's the size of a building?
Is it by any chance the 80s movie called "The Blob"?
I read quickly over a review and it sounds very similar to what you are saying, though it doesn't say anything about sucking people through their toilets in the short review.