It sounds like you might have a slightly leaky heart valve
. Our hearts have four valves, the mitral, aortic, pulmonary and tricuspid. It is relatively common for one, or more of our valves to leak. The leaks, also referred to as regurgitation (basically, backflow), can range from trivial to severe.
Because your report said you have a soft murmur and the doctor said you've nothing to worry about, it doesn't sound like you've anything to worry about.