Is there frogs living in my toilet?

For about a month now there has been awful smells coming from my bathroom. And not just after being used, strong smells of sewerage would just waft out at random. I've had problems with my plumbing before with being able to flush & according to plumbers that have come out; a drain that is used by the whole street was blocked & covered over when a new neighbour renovated their house & was covered over by builders. So all the streets sewerage keeps building up leading to problems with flushing. So along with the random sewerage smells, it has been getting difficult to flush again. Just now though I swear I heard a "ribbit" noises coming from my toilet!! Has anyone ever heard of things like this happening? any idea on what might be happening? Whats gonna happen if they just continue living there? Help!