Hmm, that is interesting. I looked up concha and it does indeed mean "shell" in Spanish ("seashell" I presume). Conchita would mean "little seashell".
However the name Concha is listed by multiple name books/websites as being a diminutive of Concepcion. No reference is made to the other meaning, which has doubtlessly had some influence. Conchita is a double-diminutive.