Why Can't My Dog Jump Up?

Lyme disease has an incubation period of 2-5 months experimentally.  It is highly unlikely that this lameness is due to a tick exposure 2-3 days ago.  Lyme disease typically causes a polyarthritis (multiple joint lameness).  Your dog can be tested by your veterinarian to confirm exposure and have titers done to assess if active infection is the problem.
Degenerative joint disease leading to arthritis is a common problem especially in older dogs that become "weekend warriors" ie are couch potatoes during the week then on weekends go for long hikes, runs, or other activity.  Your veterinarian can take x-rays to determine if this is likely.
Other concerns are intervertebral disc disease, FCE (fibrocartilagenous emboli-cartilage ie a piece of fibrous cartilage causes damage to the spinal cord), neurological issues, and neoplasia (always has to be on the list in an older dog). 
I would not give your dog medications at home without consulting with your veterinarian.  There are safer medications for dogs than aspirin and before giving an anti-inflammatory in an older pet we need to make sure the liver and kidneys are healthy.
Have your dog examined by a veterinarian, have diagnostics run as needed, and start safe and appropriate therapy.

My dog is only 3 years old.  He always slept on our bed with us and his perch in the living room is a sofa table that he had no problem jumping up on and off again.  He still runs and plays with the other dogs, but can't jump on the bed or on his lookout perch.  He's a pretty big dog.  Seems mopey too. What is wrong with him?

If this continues, I would get her to the vet, it could be fatigue, but 9 years old is getting up there, and it could even be arthritis. Keep an eye out for the next couple of days. Hope this helps, good luck.