Camketchum

I have read other threads of this bug occurring on other players files, in which a pokemon would disappear from your party in exchange for a random pokemon typically at a lower level. My Pikachu, almost level 50 dissapeared and yielded a level 15 sandshrew. I already have a level 30 Sandslash and would like to see my pikachu return as he is one of my leading pokemon. I would greatly appreciate any assistance in this issue. Also, to clarify, I have checked my PC and Pikachu is not present.