my baby is 8 almost 9 months old and has been doing the same thing, I think docsaf is right. Growth Sprurt.
Maybe it is a growth spurt? Or teething? There was a 3 week period before my DS got his 3rd tooth when he started waking up a minimum of once a night. Just like your DS, he had been sleeping through the night most nights since he was 2 months old. My DS had an awful night, a slight fever the next day, and threw up. When I brought him to the doctor, we found the new tooth. The next night he was back to his normal routine. For the past 4-5 weeks my DS has been waking up 1 night a week and sleeping through the other nights. Good luck!