Well, the way I see it, it's not much of a full on romance as it is a great friendship that Cinderpelt felt disappeared once Sandstorm became Fireheart's love interest. As an apprentice, Cinderpelt was mentored by Fireheart- personally I think she greatly admired him, and I'm sure loved him dearly. But after her accident, she wasn't able to be a warrior and had to become a Medicine Cat, who could neither love another cat, nor have kits of her own. And I think such a terrible shock like that left her wishing for something she couldn't have, like Fireheart.
But that's just my take on things.