It might be Japanese clover (Kummerowia striata),
http://plants.usda.gov/core/pr...
or its close relative Korean clover (Kummerowia stipulacea),
http://plants.usda.gov/core/pr...
but that's just a guess without the flowers.
For what it is worth, both species used to be included in the Lespedeza genus and Kummerowia stipulacea is still commonly referred to as Korean lespedeza around here