Bottom Bracket Apoclaypse...

I assume you have the old-non-sealed/cartridge type BB otherwise what follows is nor correct (i.e. you do have loose ballbearings that you can tighten the cups on to).
If my assumption is right then maybe you tightened the lockring-side cup onto the bearings too much?

1. So ...take the cranks off.
   
2. Take the lock-ring off.
   
3. Loosen the lock-ring cup until the axle has some play in it, then tighten it in just a little so that it turns freely, but no play.
   
4. Then tighten the lockting on whilst making sure that the cup does not turn (and that there is no play when you are done.
   
5. Put cranks on, should be good now .....?