Well, that not weird!! Angel does that! When I see him sniffing his butt, 98% of the time, if not 100%, he has to poop! I give him time and he WILL poop!
He is pee pad trained, but I have taken them away and want him to use outdoors! So, when I see him sniff his butt, I take him out and he poops! Not always immediately, but eventually.
But, yeah, it is kind of strange, but maybe she's signaling that she needs to go potty!