It probably is a Polygonum, the flower looks more appropriate in the last image, and there is a hint of a dark splodge (not always a reliable ID characteristic in any case) on the leaves of the first picture. I'm not sure which one it is though from the images, apparently there are 46 species in the genus in northeastern North America according to this page, which also notes that "Specific species are hard to tell apart":
http://www.bio.brandeis.edu/fi...
If you look at the nodes it probably has an ocrea, and kind of membranous sheath like this:
