chinese crested not himself?!

My 4yr old hairless chinese crested has been more inactive than usual for the last 4 days. He only occassionally shows excitement to the family upon our arrival but not nearly to the degree he normally does. Prior to this it was intense excitement until we pick him up everytime. Now he wags his tail and sometimes puts his front paws up on you then walks away to his bed. He has stopped playing at night and doesn't instigate affection as usual. His eating of dog food has cut down but he still wants table scraps. He doesn't initiate going outside as usual and does not rush back inside as quickly has he had for the 3 years we have had him. He doesn't have vomiting or diahrea. A vet looked at him and found no physical problems or parisites. We didn't do a blood test due to cost. What diseases or issues fit this description that a blood test could reveal. He had a severe case of seed ticks 6 weeks ago in MO but no problems so we think.