Here are some articles on pyometra symptoms and treatment:
Pyometra - Dogs | Thickening of the Uterus | petMD http://www.vcahospitals.com/main/pet...ra-in-dogs/915
It sounds like it's not pyometra (at least not yet), but if the discharge is different from the way it was during past heat cycles, I would definitely check with a vet to see if something is wrong.