Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am very sorry to hear about your pup Leo's rapid breathing and shaking.
Both things can be signs of anxiety, pain, fever or low oxygen levels.
What color are Leo's gums and tongue? Are they bubblegum pink, gray or white or pale pink?
If they are bubblegum pink then his oxygen levels are likely fine and he has something else going on. If they are gray or white then possible heart disease, internal bleeding, red cell destruction or lack of production are possible and he should be checked by a veterinarian promptly.
Is he eating and drinking normally? If so that is a good sign. Dogs that are in real trouble healthwise usually lose their appetite.
If his appetite is off, he's been vomiting at all or drinking lots of water, or has a tense, painful abdomen with gentle pressure then it is likely his abdomen is uncomfortable, he has gastrointestinal upset of some sort, and that needs to be investigated further.
Can you take his temperature? A normal rectal temperature in a dog is 100F to 102F, anything at or above 103F is considered a fever and indicates infection or systemic inflammation.
Is he having trouble jumping, going up or down stairs or rising from a lying down position? If so and all else seem normal then musculoskeletal disease, possibly intervertebral disc disease (very common in dachshunds) or an injury are likely behind his symptoms. If you can find a particular area that seems painful warm compresses or a heating pad set on low on that area for 10 minutes at a time several times tonight is indicated.
If this has been going on for several days a hands on veterinary examination is the best thing. I understand that it's past hours and you don't have a local emergency clinic, and that may be difficult tonight so I've given you some ideas to try and pin point a little further what may be causing his distress and the urgency of each possible underlying problem. Unless his color is very pale or he is extremely weak and has lost feeling in his legs while these symptoms are concerning they are likely not a true emergency.
I would recommend having him seen tomorrow though if he's not back to himself.
Please reply with further questions or specific responses to the questions I've asked above for further guidance.