Chihuahua bitten by something?

Who knows what it was. I would go ahead and give it a small amount of benadryl just to make sure it doesn't have a reaction while your asleep. It can prevent it from happening. It's better safe than sorry. =) I keep childrens benadryl handy for my dogs. Just be sure to give it a TINYYY amount since chihuahua's are small dogs. I wouldn't worry if it's acting normal now. If it is worse in the morning, then I would take it to the vet.