Its debated if Sayid has been "possessed" by another character. We'll obviously find out pretty soon one way or the other. For what its worth it seems unlikely this is the case IMO. Sayid has come back to life, how and why is yet to be determined.
On the issue of memory, his last memory should be the point he passed out but the same process was used on Ben and he obviously forgot that Sayid shot him (as Richard said he would). So its quite possible Sayids memory loss could go back further.