Long distance? is he cheating?(advice from men)?

[Wonderwoman]

I would like to ask you for a little advice. My boyfriend and I of a year are in a long distance relationship (4hrs away) I had a miscarriage two months ago and I wasn't able to have "you know". He didn't bring the subject up and he was perfectly fine with no sex. But he didn't say how much he missed it. I had doubts, that maybe he's cheating. I mean when we are together he looks at other woman infront of me. But just recently I checked my boyfriend's friend list on facebook to find his brother to ask how his pregnant wife is doing and in his top friends was a female. Me, thinking it was his sister because he has two sisters, I met one but not the other, I asked him if it was his sister and I would love to meet her. He said no it wasn't his sister, it was just someone he met via social media. I was ok with that answer, it was no big deal but when I tried to engage in a conversation with him after my question he had an complete addittude and he got very quite. Before I didn't think he was cheating until his quietness and addittude. Do you think he was flirting with her or talking to her, she lives in the same city as she does. I sent her a message to find out, I didn't ask her- I just engaged in a conversation. I told my boyfriend I had spoken to her and loved the things she wrote on her wall...he got so mad and went off. Then stayed quiet. I don't understand, I wouldn't care if he had spoken to a male I had as a friend on facebook...because I know there is nothing going on with me and this guy. Can you please explain his anger and do you think I'm being paranoid? Do you think he couldve been flirting with her?