Doesn't happen often, but it does. I had a friend who had 7+ interviews in the first app cycle and didn't get into any schools. You were probably a very strong candidate on paper to have 4 interviews by October, so it might have been your interviewing skills. And if you're a reapplicant to some of the schools then you need to show them significant improvement. New PS, letters, extracurriculars, etc.
It's normal to feel alone and pessimistic, but evaluate your apps again. It's not too late to apply to DO schools either for a backup plan. Granted no one wants to go through this more than once, but I do know of people who applied 3 or 4 times to medical school. They persevered and eventually got in. It sucks, but if you want to be a doctor then you have to do whatever it takes right?