aggressive biting

Question
I have a seven month old female Husky/Terrier cross puppy. She usually picks things up in her mouth and doesn't have a problem when I remove them. However, she has shown possessive behaviour towards these objects when my adult daughter has approach her. Anyway, I had her spayed (while in season -vet said it was ok) two days ago. After I had taken her outside last night for a call of nature, I tried to take something she'd picked up out of her mouth. I was shocked when she bit me really hard, in fact I had difficulty removing my finger from her mouth. Needless to say, I'm now afraid of her, the strange thing is that she is  submissive and placid the majority of the time. I've grown up with dogs and am now 48 years old, but I have to say yesterday was the first time I have ever been bitten so badly.
I'd be grateful for any advice.

Answer
Your Vet WAS WRONG............FIND another one!

Spaying a bitch in season is a HUGE no-no; the hormonal balance of going through the entire heat cycle is absolutely necessary once the bitch has been allowed to go into a cycle.  Additionally, your bitch may have made an association between the pain she is in (and IT IS painful -- especially since her tissues are inflamed from being in heat!) and approaching hands, bending over, etc.

You may very well have set this dog up for possession aggression by forcibly removing objects from her mouth in the past.  If she has a dominant personality (quite likely with that breed mix), the manner in which to teach a dog to relinquish objects is to play "take it, leave it" during normal play routines.  One offers the dog a toy, plays with the dog for a few minutes, says "Leave it" and removes the object (while keeping it in sight) until the dog stops trying to obtain it, then offers it back to the dog with a "take it" and continues play. One repeats this game for short periods over several weeks. Keeping the toy at the end of the game is very important when dealing with a dominant or possessive dog.  The dog learns that "leave it" means YOU have the right to it, it's a sign of rank and psychological dominance.

The combination of hormonal crisis, post surgical pain and the lessons your dog has learned about using aggression with your daughter has created a potentially serious problem.  You need to find a credentialed behavior expert who can assess your dog's temperament, do an in person evaluation and an extensive interview with you, and offer you a plan of behavior modification to see to it that this dog does not develop any further behavior problems related to aggression.  Call a veterinary school in your area (even hundreds of miles away) and ask for a reference to a Ph.D. or DVM animal behavior expert, who should come to your home (not see the dog in his/her office.)  Do NOT show fear to this dog.  She will not understand, it will frighten her, upset her, and cause other problems.  It's highly unlikely this dog will suddenly become Cujo.  Remove all her toys; offer them one at a time for short periods; after a half hour while she is away from the toy, remove it.  Do not begin playing "take it, leave it" until she is fully recovered from surgery.  Find another veterinarian NOW and discuss short term hormonal therapy.