My female chihuahua is acting strange? she goes upstairs and stays there? she never has done this? withdrawn?

our female chihuahua has been in heat for 3wks and seems too be finished, but now her behavior patterns have changed? shes withdrawn and has no energy it seems? she used too always hang out on the 1st fl of the house and for years shes done so.... but lately in the last few days she goes up too the 2nd floor by herself while everyone else is on the 1st floor? when we call for her she doesnt come down like she has always done before? shes a great dog, fun loving/loyal but she seems very depressed.... at first i was worried she was poisoned or maybe ingested something as she seemed so drained that it scared us. im wondering if this is the post depression at the end of her being in heat? what do you think? what do we do too make her feel better? ive read about chewable kids aspirin, would this help any physical discomfort?