Does your dog have a heart murmur?
Is your dog possibly painful? Especially in the back?
Has he been around other dogs lately?
It's hard to determine what's going on without a thorough physical exam....... If this behavior persists tomorrow, I would highly recommend a vet visit.
A few thoughts...
1. Your dog may have pneumonia. It can cause a fever and the clinical signs that you are describing.
2. Your dog may be painful. Often they will pant or breathe heavier than usual if they hurt somewhere.....ANd they may also stop eating.... Maybe he has an issue with his back hip since that is twitching.
Tonight, you can try some aspirin-- either one baby aspirin or 1/4 of an adult aspirin every 12 hours.
Obviously if this is a pneumonia then your dog may need antibiotics which your vet could prescribe.
I hope that this helps you decide what to do tonight..... If you are really concerned, you could take him to a vet tonight. It may give you peace of mind.