Not sure what it is. I suspect it's some kind of ruellia, but I will defer to the experts at the South Carolina Native Plant Society and wait for their reply. It's not sharp - had to shoot it from a distance in the weeds.
Update from the SC Native Plant Society and Dr. Hill: He's says it's Petunia x hybrida and an escapee from someones garden. A hybrid cultivar but, much smaller than any petunia I've seen and the scalloped edges are very unusual.
@wakelys Thanks, Susan!
@samae Thanks, Sarah!
@ludwigsdiana Thank you, Diana!
@kjarn Thank you, Kathy!
@lesip Thanks, Leslie!
@craftymeg Thank you, Margaret!
@framelight_byasli Thank you, Asli!