Per my agreement, child support stopped at age 18. Is it possible for my x to take me back to court in order to continue child support? I am in the State of Maryland.

Sure its possible for your x to take you to court. Will she win? Unlikely. If the support agreement says it stops, it stops. She MIGHT get it extended if they remain in school.

You Should No Longer Be Resposible For Child Support, Unless Stated In Your Parenting Plan That You Will Help/or Be Responsible For Paying College Tuition. That's The Only Way That You Would Still Be Responsible For Something.