Okay, if you added IK after thats cool. If you did it before its a case of having double transforms in the hierachy, Ik RP chains can cause this problem

If you painted weights and mirrored those sometimes it doesnt work properly so give it another go. Theres a bit more to painting weights than just selecting skins and painting on them.

Hold the weights on all joints except the ones you want to paint weights on. Sometimes some verts need to be tweaked in the component editor after as well because some joints may be sharing the skin. Its worth noting too that use the 'add' command only as a rule with a setting of . 2 to start with, this is ideal to use as well if you have a tablet, as you can use the pressure setting to allow for a more controled effect over the joints being painted.

Cheers
Jay