This may begin as a reddish-brown. As the heat cycle progresses, it usually transitions to a pinkish-red. The first heat may produce a clear to pink discharge. For toy breeds dogs, the flow is rather light and may go unnoticed for some. For others, the flow will be heavier and the use of a canine doggie-diaper will come in handy.
When the female is receptive to the male during the Estrus phase, she will be flirty with males and raise her tail to them.
As the heat cycle ends, even if a female has had no contact with any males dogs, she may show nesting behavior. During this time, she may behave in a mothering way to toys (most often soft stuffed animals). It is important to allow her to carry out these activities as they are harmless and brought about by hormonal changes.
It is suggested to not touch what she may consider her impromptu 'litter'. As she enters the anestrus stage (resting period between heats), this behavior will go away. Though, it is normal for dogs to have favorite toys and be protective of what they consider to be their belongings.
Mood Swings and Behavioral Changes -
There can be some mood swings
when a female is experiencing heat. As the uterus contracts, there may be some cramping that causes a dog to retreat. There may be some minor whimpering as well.
During the estrus phase, she may appear to be moody; sometimes acting reserved or sometimes behaving agitated or anxious. This type of moodiness is temporary and a dog's normal disposition will soon return.
A dog in heat may have such strong urges to mate that she may try to runaway, even if she has never previously shown such behavior. During this time, owners should be very aware when opening doors that lead outside. As always, keep your Chihuahua on leash whenever outside.
Males dogs will know that a female is in heat and can actually sense this from up to 3 miles away. Each time that a female in heat urinates, she leaves behind traces of blood that send out very strong signals to all un-fixed males in the area.