I have heard of it and there are a couple things it could be. It could be the deodorant you are using. The way to fix this is to make sure you scrub (with a loofa) under your arms and switch deodorants until you find one that doesn't discolor your armpits. Another reason could be the beginning signs of diabetes. If changing deodorants and scrubbing your armpits doesn't work than think about going to a dermatologist.
After puberty it is natural for the skin to darken a little, and with shaving the hair makes the armpit look a little darker than the rest of the skin also. If it is not normal there is a good possibility that it could be fungal. A topical fungal treatment would take care of that easily, but don't take my word for it, go see your doctor to get a first hand diagnosis. The odds are that it is nothing serious at all.