Don't stress about one B. Lots of Bs would start to become a problem but you're so early-- you'll be fine. I got three Bs (I think... maybe 4, and one B-), all in science classes, and so far have three interviews at good schools.
Most med schools do count AP credit if your university gives you credit for it on your transcript. I got AP credit at my university for intro chem 1 and it hasn't been a problem.
Another option besides a masters or prolonging undergrad would be taking at least a year off to work after you graduate, especially if you'll be graduating young. You could work as a medical scribe, do research, or do something else that interests you. I can say from personal experience, it's really nice to be out of the pressure cooker of undergrad while applying to med school. You'll know yourself and your goals better, you'll have more opportunities to work with and get to know people who aren't in academia/pre-med, and you'll just have a wider variety of experiences to draw from when you go into the application process.
Also have fun in college!