I believe this is bk 3 in the Highland Brides series. Back blurb:
Haydan MacGowan is known as the Hawk-- the strongest and most dedicated warrior of the king's guard. A man who shares the secrets of his heart with no one, he is impervious to feminine wiles-- until luscious temptation arrives in the lovely form of the gypsy lass, Catriona. But even as the Hawk falls under her spell, he knows she's hiding something.
Catriona is on a desperate mission-- and the Hawk is a very large, very handsome obstacle. Seduction is her best weapon, she thinks-- until she gets caught in her own trap. And as passion burns bright, she wonders: dare she trust the man she's come to love? Or must she betray him?