Thanks for answering my questions.
With an unaltered female who is lethargic and drinking more than usual after a recent heat cycle, the first thing we think of is a pyometra.
The term 'pyometra' means pus in the uterus. This pus formation in the uterus results from infection, hormone imbalance or mucous buildup inside the uterus. Most dogs and cats suffering from pyometra are presented because of loss of energy, increased thirst and poor appetite.
Pyometras can be either open (where the cervix is open and there is drainage) or closed (cervix is closed, no drainage, and dog is very sick). If your dog is licking her vulva I suspect it is open, but there is always the chance it will close. If she's not paying any attention to her vulva at all, then it's probably closed.
With an open pyometra, you probably wouldn't see anything on x-rays as the pus is draining out. An ultrasound would likely show a thickened uterus and fluid, but the x-ray isn't that good for detail like that. An open pyometra also might not show any changes in bloodwork, particularly early on. With a closed one you would expect an increase in white blood cells, but not necessarily early on.
My suggestion is to get your girl to an emergency vet as soon as you can. If she does, in fact, have a pyometra and it is left untreated, it could very well rupture and kill her.
I hope this helps.