Hi, how old is baby and how is weight gain? Are you pumping for any reason? Is baby nursing exclusively or being supplemented? if applicable, please explain about any pumping or supplements if you can.
This sounds like a bleb. They hurt like a bleep. A bleb or a plug is not going to prevent milk coming out necessarily. A friction blister is another possibility. Is the pain you are having stemming entirely from the this area, or have you been having more general nipple/latch pain as well?
Green poop is an entirely normal variation in poop color. I repeat there is nothing wrong with green poop assuming baby is healthy and gaining.
Fast letdown is only a problem if it is causing a problem. Some babies love a fast flow and others struggle with it. If your baby is struggling with the fast flow, here are the tried and true things that help:
Encourage baby to nurse frequently. At least 10-12 times in 24 hours, more often is fine. If baby prefers to nurse one side at a time, fine. Let them.
Lean back to whatever degree is comfortable when nursing, with baby positioned to some degree on top of you.
If baby is having trouble when the milk starts to flow, take baby off and let the milk go into a cloth for a bit before you re- latch baby.
Info on blebs and plugs:
http://www.llli.org/docs/00000000000...plugsblebs.pdf and
http://kellymom.com/bf/concerns/mother/nipplebleb/