Being obsessed with poop is a prerequisite for Yorkie moms.
The lamb food probably did not have more fillers, but may have been higher in protein/fat/calories, causing more frequent poops. You can look up this info on the internet to compare the foods' percentages.
The fact that she is eating less may also be the reason for the fewer poops. Having it get stuck to her rear end is a little concerning and may indicate she is slightly dehydrated. This all could have coincided with warmer temperatures too.
Usually firmer poops are more desirable, but too firm is undesirable.