I thought I had a tourist visa for 90 days in Mexico and it turns out its only for 30. What can I do?

I've been living in Playa del Carmen, Mexico now since December 19, 2006. I am going to be here until the middle of March. When I entered Mexico, I thought they would give me a tourist visa stamp for 90 days and it turns out it is only for 30 days! I am very scared that when I leave the coutry, they wont let me leave. What can I do? Will they let me leave? Will I have to pay a fine and then they will let me go? Has anyone had this problem before? Thanks so much in advance!