I'm pretty sure this is Boucing Bet (aka Soapwort, Saponaria officinalis). But that's supposed to have 5 petals, & this one has 6! I have a shot of the leaves I can upload, if that will help.
Large, showy, weedy, smelly, & poisonous.
Common Morning Glory (Ipomoea purpurea) -- an escaped ornamental, common in meadows at this site.