>Could this be the start of a dominance, pack leader thing?
No. Please put this outdated theory out of your mind. Dogs simply don't plan like that.
There are several possibilities; maybe she doesn't need as much food as you think (I have a dog who'll decide not to bother with his meals every so often. He's come to no harm); maybe it's the wrong food for her, and perhaps gives her indigestion. Most likely is that she's unsettled following the loss of your old dog; one of mine lost 2 kilos in two months after her mother (definitely 'top dog') died and has only now, 7 months later, regained the weight.