Is she trying to get comfortable? When you go for your post whelp exam (24 but no more than 48 hours after) be sure to mention that. It is a regular part of the exam to check for things that are retained to cause infection (like placenta or decaying pup) but I might worry. Were there fewer pups than on the ultrasound or x-ray or did it match exactly (sometimes there is a smaller one hiding)? Hope your girl is okay and good luck.