[Ianto immediately goes on red alert at the expression on Jack's face, because he knows for a fact he's happy with how their relationship is. At least, he thought he was happy. Or maybe he's missing something or he's said or done something without realizing it, or any of the other millions of possible scenarios his mind immediately starts concocting.]
At least, I thought so.
[There's a question in the statement, wanting very much to hear that he's overreacting and its all nothing.]