So you went through a nether portal... and wound up far away in the overworld?
It is one thing to go through a portal, then try to return and wind up a bit more than 1k blocks away from where you expected. That's a normal consequence of having other portals nearby and vanilla nether portal mechanics.
http://www.minecraftwiki.net/wiki/Nether_Portal#Portal_Linkage_between_Overworld_and_Nether
Sometimes that is fixed by breaking all portals except the two you want paired, sometimes not. On a server with lots of other people making portals it is best to give up on predictability and just bring a way to teleport home.
If you really went through a portal and immediately arrived at a different part of the overworld... wow.
Yep. I managed to travel through about 6-7 different portals in the overworld and ended up -9999999+ away from home. Going back through each portal just put me further and further away. Just to clarify the original one I made was the very first made.