Here's my advice for what it's worth.... I would go to her the next time you are on and tell her how much you love your job and enjoy working there. Tell her you are not sure why she has the impression you are not happy but you will definately try to put a smile on your face and that you really want her to know how much you value your job. By the way, you haven't complained to anyone else have you? Because maybe someone is repeating anything you might be unhappy with.
Then, I would screen your calls when you are not at work and help out when you can or feel like it or are on call but if you don't feel like it, you can just say, I'm sorry I wasn't available. But I would also pitch in on occasion and definately when you are on call.
Good Luck!