Help Appreciated

Whats up everyone? I'm new to the forum and surfing as well. I live out here in south Florida and am basically trying to teach myself how to surf. I had a quick question, I've gotten up a couple of times, but yesterday while surfing something that had never happened before left me with a bruised shoulder so I was wondering how it happened. As I was swimming to catch the wave the board sort of just gave out from under me and I know it happens if I'm too close to the front but in my opinion I was perfectly balanced in the center. But I ended up hitting the sand shoulder first. Any help would be appreciated. Hopefully this isn't a stupid question.