My tranny has a real bad leak right now and im in the process of fixing it..mine, when low did the same thing and poped out of 3rd gear and when I filled it back up it didn't do it...then when it leaked out again it did it again so its a possibilty that your tranny fluid is low but, don't mark my word on it. Could be a number of reasons why its doing it.
The leak on my tanny is coming from the blehousing..we took it to the auto shop that installed my clutch and they put it up on the lift. The guy said the front seal on my transmission is #@%#@%#@%#@%ed and that its pouring gear oil over into my bellhouseing but, thats just a guess. The leak its self is coming from the bottom of the bellhousing right between the cover for it and the belhousing itself. So for me it could be anything. My freind and I are taking out my tranny and we are going to replace any seals or bearings that might look messed up. For your sake I hope its just the nut to drain your tranny is loose or something easily fixable . But, if its leaking like #@%#@%#@%#@%, like mine is then it might be something inside the transmission. good luck.