Hi- This happened to me awhile back during some clothespin play (in fact, I think I might even have started a thread about it, just like you!). I went to the gynecologist and told her what had happened (including that I was playing with clothespins- no sense in lying). She said that if it is just a drop of milky or clear discharge, it doesn't continue, and there is no pain or change in the nipples, then not to worry, as it was just the nipples doing what they're supposed to do when sqeezed and sucked on (namely, produce milk, which can happen in small amounts even when not preganant).

However, she said that if the discharge is white, opaque, colored, bloody, painful, continuous, or if there is any change in the nipple see a doctor immediately, because it could be an infection at best, or a sign of cancer at worst. Also, when you see a doc, be sure to tell him or her exactly what you were doing that caused you to notice the discharge- that info could be vital to diagnosis and treatment (as in, it might be normal for a nipple to leak a bit when squeezed for 20 minutes with 20 pounds of pressure, but abnormal for it to start leaking completely randomly).
-Phantome