What it mean if your spouse keep love card send from someone else but not you?

It all depends on what situation ur marrage held either love or arrange marrage.
By ur question i think u may had a love marrage & basically if there are no problem issue in ur relation with her then according to the human nature people(either a girl or a boy) likes to keep a thing which seems to be unusual to them,just as a thing(love card,etc.) which make them feel extra special then other or just as a remberence of other important people in their life,i don't say that she don't think u an important person in her life but i think that she should been much useto to ur letter & that also doesn't mean that she do not love u,she might be loving u almost as she had married u and not the other one who send her cards.
i can say that life change after marrage and the couples sometime feel reatless to each other,just keep in mind that u should always have faith on ur patner & should be co-oprative & open-minded towards ur life patner

well it must have meant something special to her it doesn't mean she is cheating it probably is just a memory so to speak probably nothing to worry about did she keep any of your cards?