The only reason to stop nursing would be becuase you are prone to "preterm" labor or have other medical reasons that would need to be considered if you are nursing while pregnant. Let your OB/Gyn know that you are nursing and discuss with him/her if there is any MEDICAL reason you, personally, should stop nursing. For all normal, low-risk pregnancy's, there is no reason to stop nursing the entire time you are pregnant. Many of my pregnant mommy friends have nursed while pregnant and then actually tandem nursed their children! Very cool! Kudos to you for nursing your baby for so long!
As for getting pregnant while nursing: if you are having menstrual cycles, then nursing while trying to conceive should not be a problem. If your cycle is still a bit out of whack or not regular, it may be more difficult to become pregnant, but it is still very possible to conceive. If your cycle is not regular, you may want to try an ovulation kit to help you determine when you are ovulating. This would be very helpful and could save you some frustration.
As always, it is best to talk to your OB/Gyn about these issues as he/she knows your medical history and can help you determine the best course of action. In the meantime, I hope everything works out and you are able to continue nursing and become pregnant...The best of both worlds! Good Luck to you!