Typically, child support orders provide that the obligations continue until the child turns 18 or graduates from high school, whichever is later. The specific language of the divorce decree/child support order will control the child support obligations so you should consider having an attorney review it in order to give you a definitive answer.
It sounds as if your decree has the standard language. If it does, then the child support would stop at graduation. I cannot stress how important it is to review and understand your entire divorce decree. Look for anything which might indicate your liability for college.
Contact a lawyer, forward them the order, and pay them a small fee to look at it. Couldn't hurt.