I have a chihuahua x Pomeranian, shes 10months old and about a week into her first season.
She's very clean and had no mess anywhere which is brilliant, but I've noticed a massive change in her personality. She's very quiet, very lethargic sleeps about 23 hours a day!! and hasnt wanted to play with any of her toys. She noramally always wants us to play fetch with her.
I'm really worried about her as this really isnt like her.
My girl on her first season would not tolerate the car engine running - thankfully she grew out of it!
She probably wonders what is happening to her.
If she suddenly goes off her food/water and/or starts being sick and depressed/off her feet then get her to the vet as soon as possible as they can develop a condition called pyometra that is life threatening.
If you are really concerned then it may be worth a visit to the vet just to put your mind at rest.