7 week old crying in her sleep...?

[xQuinnx]

Sometimes my 7 week old will cry in her sleep sometimes before feeds but usually after. This seems to happen more at night when it does happen, she wi feed (sometimes half asleep!) burp and then fall asleep. When I put her down she will be fine for a few minutes but then start to wriggle and make strange noises, kind of like a cross between grunting and snorting, sometimes she cries a little but she's fast asleep. It only goes on for a few mins but I'm not sure what's causing it? She's now exclusively formula fed, up until a week ago she was formula and expressed milk fed, I not sure if that has anything to do with it.