I would not worry too much unelss you see a rash or sores. It is summer time and your puppy is small. Her belly touches the grass when she is outside and the blades of grass can be slightly irritating to the skin on her tummy. If there are no bumps or sores, she is probably just reacting to the grasses. Keep an eye on it and see if it gets better as she grows up.
Sabra - female GSD - born April 30, 2012
Sargeant - rescued senior GSD male/foster failure - approximately ten years old (August 2013)
Max - rescued GSD - Dec 22, 2010 to March 7, 2012 - at the bridge
Sable - rescued GSD - at the bridge