I assume you changed races to the DIathium. In which case you probably went down in HP due to their lower multiplier.
Your HP Is 236.
The 313 is what your current HP is, if you engaged in combat you only be able to heal to 236. For some reason, anybody who had higher HP before changing to Diathium retained their characters current HP at the time of the change. Think of it like a health boost till someone shoots at you or a creature swings its tail.
It's actually still a bug. It seems to recalculate your strength and xp levels and add the HP for those on top of the HP you had before, for the current HP.
I know Eric Jackson mentioned this in the past and his report was forwarded to Zhao, and that Zhao didn't consider it a big deal before the additional strength * level HP thing was known.. but it might be a good idea to bug base this if it isn't already since the current HP ends up a fair bit higher than it was before, and that could be problematic in some future circumstances.