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Some kind of Aster:
https://en.wikipedia.org/wiki/Aster_(gen...)
Yep, definitely sounds like Purple Loosestrife. And your pictures are gorgeous, by the way!
It's some variety of Tick Trefoil:
https://en.wikipedia.org/wiki/Desmodium
Looks like Purple Loosestrife to me as well, but it might also be Winged Loosestrife, which is native. You can tell the difference in the details of the stems and leaves, which aren't included in your pictures. See the "Comments" section at the bottom of these pages:
http://www.illinoiswildflowers.info/weed...
http://www.illinoiswildflowers.info/wetl...
Pictures of the foliage or a more extensive description of the plant and its location would help.
