What I don't like is the implication that you can be 100% sure of anything. You can't, unless you can predict the future. I mean, I suppose you can say that in this point in time, you don't have any significant doubts and feel that your heart is leading you in a particular direction, but being 100% sure, in my mind, means to be all-seeing, which none of us are. Your state of commitment changes throughout the process and most people do sense themselves wavering from time to time. This is natural. However, if you have the heart for this path, you'll recommit. Medical school and residency are tough and it'll bring out some interesting parts of you. Honestly, I thought I was a tough person, but in reality, there still many parts of me that I haven't yet faced. There's nothing like a good challenge to bring out those parts. Medical school will test you. If it doesn't, then you aren't making the most of it.
Anyway, don't let people make you feel bad for having doubts from time to time, or for feeling scared. As long as you know that this is where your heart is leading you, I think you'll be fine. It is better to face your doubts and your fears rather than pushing them aside, or burying them, where they can come out at you unconsciously. And believe me, we all have them, whether we realize it or not.