What causes a Linksys router to start blocking the ports on its own for my connectivity to the internet.?

I have a Linksys router that will all of a sudden just start blocking the ports on its own for my connectivity to the internet. Does anyone know why it would do this on its own and why?|||it's not blocking ports it does usually happen when you download and happens a lot faster when more people download trough the same router because router uses look up tables for every connection and when you download they fill up very quickly.