Relax, you have nothing to worry about. Your question reminds me of how I felt my sophomore year.
First off, don't you ever plan to stay at school during a summer session? Or take any summer classes?
If not, and if you know what your major is now and you don't switch, graduating on time will NOT be a problem. I don't mean to sound harsh, but if a load of 15 credits a semester is too tough, then you might want to reevaluate pursuing something as challenging as medicine.
I doubt that's the case, however. Look, here is the deal. We are young... there is no reason to rush medicine. Say you graduate in four in a half years... that leaves 8 months before med school to travel the world... do a study abroad maybe... work a cool job, live your life.
All in all, the average matriculation age for medical schools now a days is a lot higher than you might think... i heard it's like 24 (but it varies from school to school). It's surely not the young, virgin age of 22! What does that tell you? Well... people are going to medical school with life experiences... so join the club. It helps your application status also
Good luck
Leon