I know my doctor told to replace them every month, but I usually wear them until they bother me. I wear 30 day lenses, and often keep them in for 2 or even 3 months. If I don't have a problem, is this okay?

Wearing your lenses too long, especially overnight, puts you at the highest risk for the most severe type of infection, a pseudomonas aeruginosa corneal ulcer. Unfortunately, you can get scarring as a result of this infection that may permanently impair your vision. Try to follow your recommended wearing schedule. The idea of using frequent replacement contact lenses is to discard the lenses before they give you any problems.