Your academic record is fine; you'll be a very competitive applicant. Make sure you write a good essay explaining why you want to do medicine; that will be the key to overcoming the lack of volunteer work. I know people who have been accepted there without any medically related volunteer work, so that's not in itself a barrier to entry. At any rate, you'll never know unless you apply.
I don't know you or your reasons for making this switch, but I'll venture to suggest another possibility to you as well: take a year off, do some volunteer work, and make sure medicine really is for you. Penn keeps a database of alumni, and many of them are happy to talk with a prospective premed and/or let them shadow them for a day. This is another great way to find out about medicine. The reason I suggest this is twofold: one, it will help you figure out for sure if this is what you want out of life--it would be terrible to spend another year in school, only to again find out it's not for you. And secondly, having some experience with medicine will allow you to more clearly articulate to post-bac programs (and later, med schools) why you want to become a physician.
Good luck.
