New signs of aggression in my female Chihuahua

Question
I have a 3.5 lb Chihuahua that is a year old she was
recently spayed she is now showing signs of aggression
towards our 2 males (a 5 lb Pom who is also fixed and the
baby a 1.5 chihuahua who is 3 months) she now mounts them,
and snarls at them both.
She has never shown this type of behavior in the past,
although I believe she has always been more of the "alpha"
of the 3, she has always been playful and slightly mellow.
She has also gotten into a new habit of pushing the puppy
with her nose sometimes HARD, she also attempts to bury him
with blankets, clothing from the laundry basket or if I am
holding him clothing that is on me.
I have no idea what this means!
I have had all the dogs from the time they were 8-12 weeks
old they are all healthy (have the vet bills to prove that!)
and exercise daily.
Any ideas?
Thank you!

Answer
Did your Chi have an estrus cycle before being spayed?  Smaller breeds often come into first estrus at an older age.  If she was spayed without an estrogen level evaluation, this behavior may be directly linked to hormones.

I strongly suggest you take your Chi to a veterinary behaviorist.  Progesterone and synthetic progestins are commonly used to treat hormonally related behavioral issues.  Check the following sites or call the veterinary college in your geographical area:
http://www.veterinarybehaviorists.org/
http://www.avsabonline.org/avsabonline/

Meanwhile, what you perceive as aggression appears (from your description) to be the result of high anxiety and perhaps physical discomfort.  Mounting is not necessarily a dominance behavior; it can be an attempt at affiliation, an attempt to diminish anxiety or problematic behaviors in the other dog, etc.  Snarling, while it appears frightening to a human, is a statement that other dogs fully understand.  Actual aggression involves "attack" with intent to harm (and resultant injuries that require veterinary attention.)  Your Chi appears to be "nesting" the puppy.  Her behavior seems to me strongly driven by hormonal cause.  Find a veterinary behaviorist.