When you mention chest pain, a doctor will usually check out your heart whether you think it could be involved or not. I'm sure she listened to your heart and checked your pulse. Did she do labwork, did she do an EKG, other tests? Those tests would most likely show if you had a heart problem.
There are many things that can cause pain in the chest. The heart is one of the worst, so it's what people automatically think of. It's the scariest. Most of the time in otherwise healthy, young people who aren't at risk for heart problems, it's something else entirely. Your chest area is full of muscles that you use constantly. You can strain one without knowing it and this can cause pain. Congestion or infection in your lungs can cause pain, and that can be from as little as a cold at times. You have other organs in that area too that can cause referred pain felt there that can feel like it's originating there.
I would think that if a young healthy person developed sudden mild chest pain with no other symptoms, if it were a heart problem you would probably already know it by now and be in the hospital. While there is a chance it's something, I would believe your doctor and try not to dwell on it. The more you worry, the more your muscles tense up including the one you strained to cause the pain, so you can feel it more and then think it's something bad so you worry more, etc.