No one seemed to know what my mystery flower is, so I took matters into my own hands and looked up wildflowers on the internet. I have Spotted Touch Me Nots. They are called that because when you touch the seed pods they explode (I've never done this, will have to try). Also, the juice from the stem is known as a treatment for poison ivy, which I currently have on my arm and grows rampant in our yard (the original owner didn't like kids cutting through his yard, so he let it grow out of control. Yay, original owner. My poison ivy sensitivity would like to have a word with you, and the shingles they triggered a few years ago).
So, it really is a delightful flower which will spread like a demon but is really easy to pull out in places you don't want it. I love it and any shady spot in your garden would love it, too!