Dog Panting Shaking Being Clingy

[clewis78]

Our dog, an eleven year old boxer mix, started acting funny the past couple of days. She wants to go outside, but then doesn't want to come in. She will just sit (even though it is in the single digits temperature-wise) and has to be carried in. Inside, she has become very clingy (won't leave our side), pants a lot, and shakes. Any clues from anyone will help. Waiting for Veterinarian to open to make an appointment to take her in, but would like to hear advice in meantime.