Yoghurt

In the realm of the completely random and quite mundane:

Okay, so... you know how plain yoghurt gets that nasty weird greeny/yellow liquid in it if you eat half the container and then leave it in the fridge for a couple days?
Well, the other day I thought about eating some, so I stirred it up, but then it was SO liquidy that I decided I didn't want it. So like the lazy bum that I am, I left it in the fridge for the next several days... and today, while rummaging around trying to find a good mayonnaise substitute, I reach for the yoghurt, only to find that it has more than doubled in volume! It is the same one, because we haven't bought any more yoghurt, and it's not all gel-like like the new stuff is (you know what I'm talking about, don't pretend).
So, long story short... my yoghurt is sorta freaking me out a little. I know there's a live culture in there, but in chem they really hammered "preservation of mass" into my head. And, while there may or may not be more mass (I didn't weigh the thing) there is WAY more volume.
And I was remembering friends telling me that their mothers had made their own yoghurt by getting whole milk and some left over yoghurt and letting them sit for a while... is that possible with my poor little week-old tub of freshly-stirred yoghurt?
Only the internet can tell me. xD But I'm a little intimidated by the idea of searching in forums.
So I'm gonna just assume it's perfectly natural and safe to eat. O.O
Talk about bizarre foods.

peace.
PP.