african grey (Chula)

I have a Congo African grey. He is four years old. He was so nice and tame that he would be on my lap and crawl up the front of me and cuddle in my neck and give me kisses and all that. I had to have my hip replaced and had to leave him in the care of my sister, for about three months. When I returned home he was really nasty with me. He wants to bite me all the time. I kind of wish I could get him back to the way he was before i went to the hospital, but I don't exactly know how to do that. Would really like some advice.
Lee Beddows