I would suggest speaking with the school asap. They will work with you! If you end up getting a D, you might just have to take a college course this summer. If that happens, I suggest gathering all the needed materials such as a course description and get it emailed to the appropriate department head. First and foremost I'd contact the admissions people. Also, I would check your acceptance letter and see if there are any stipulations written...something like, "as long as you pass all your classes with a C or better, and you maintain a 3.0 GPA" sort of clause.
Like I said, I would contact the admissions people asap and explain your entire circumstance. Just be honest with them and tell them the truth.