All operating while impaired charges in Michigan are serious, but even more so for people who are underage. If you are going to consume alcohol before you turn 21 years old, you are facing some very serious charges if you decide to drink and drive. The same is true if you use drugs and then drive.
Your boyfriend gave you his credit card to do some shopping. You had his full permission and did not spend over the amount he specified. As far as you knew, everything was fine and you were not doing anything illegal.