WOW!

That will certainly make you wonder what happened. First off, I doubt you would lose that much oil without getting some water back in to the lower unit. Was there any water in it when you drained it? If so, you've definitely got a bad seal somewhere. More than likely the rear prop shaft seal. If there was no water in the oil then there may have been a miscalculation when you put the new oil in. Maybe you thought you had it full but you didn't.

Bottom line, if the oil was clean fill it up again and run it a couple of times and see what you come up with when you drain it. If it had water in it then it's time to replace some seals.