chihuahua displaying unusual behavior

Question
My 2 yr old chihuahua Tinkerbell has been displaying rather unusual behavior. She is normally a very happy dog who cuddles up next to anyone sitting and especially if you have a blanket. Lately she is snapping at anyone who tries to touch her, even me and i'm her favorite. She goes into the pin only if everyone is leaving home and that's rare. When we lift the pin up she normally runs out immediately, however for about a week she has been hiding under her blanket and refusing to leave the pin. I came home from work and went to the pin and lifted it up and she would not move. I reached in and tried to pick her up and she tried to bite me. The next morning I lifted it and she came out and went potty outside as normal but when she came back in she when and stood back by the pin, i lifted it and she went back in. It is very unusual for her to snap at me, when i reach for her she normally turns over on her back so i can pick her up. She is hiding under the blanket and not wanting anyone to touch her and I can't understand why. Do you think I need to take her to the vet.

Answer
A dog that rolls over to expose its belly IS NOT NECESSARILY making an invitation to be touched; this is a sign of abject submission and it must be respected.  When the dog rolls over, WALK AWAY; first, lick your lips (a stress reducing signal), break eye contact, and walk away.  Never reach for a dog when it is in hiding; you are setting yourself up to be bitten.  The dog is afraid; this may have been inadvertently caused by something you did (totally without meaning to, of course!!) It may be associated with something that has nothing to do with you at all (a dog can acquire a conditioned fear response quite easily from what appears to us to be the most benign thing, such as the sound of dog cookies rattling in a box!)  Your dog may have demonstrated submission and now is demonstrating defense aggression because her signals of submission were ignored (you didn't know any better, but she doesn't know that.)  She has lost trust in you.  I think a visit to the veterinarian is a very good idea; you can rule out physiological cause (such as pain, eye involvement, neurological problem, etc.) and the veterinarian may offer a medication for short term treatment of her extreme fear.  Meanwhile, you need to learn about dog psychology (read John Fisher's "Think Dog") and dog communication (read Turid Rugaas' "Calming Signals") and positive reinforcement training (read Patricia McConnell Ph.D.'s book on this topic.)  All these books are available at fair price from Amazon.  Carry tiny tidbits of treat (chicken hot dog, fat free cheese, etc.) and whenever the dog approaches you voluntarily with no sign of fear, toss a treat and praise her.  Keep your eye on her and praise her for every effort to interact with you.  Don't pick her up, rub her belly, pat her head, force her to go anywhere by coercion.  You don't want to encourage any more aggression.  I don't know where you live, but an actively biting dog (even from fear) is a problem that requires a certified applied animal behaviorist. You can ask for referral from your veterinarian or contact the veterinary school in your geographical area for referral.