I'm pregnant with my first, and just had my 16 week appointment this morning. Heard the heartbeat loud and clear!

Is it possible for me to be feeling the baby move already? I swear I felt a little flutter, almost like a muscle twitch. Doctor said I could just feeling gas bubbles, but this felt different than gas.

Most websites say that first time mothers usually don't feel anything until 20+ weeks unless they are thin. I'm tall (5' 11") and at a healthy BMI but I wouldn't consider myself "thin". Am I imagining things? What's your experience?