I pulled out the clubs earlier this spring while on vacation, about 8 months post surgery. My advise is to take it slow and build up your endurance.
I'm more of a baseball/softball player than a golfer (as my golf game demonstrates), and in late March I started hitting the cages as I always do pre-season. I had been working out with weights, walking and biking on a regular basis prior and I thought I was ready to get back 100%. After hitting about 75 pitches on a beautiful Chicago early spring day, I went home happy about swinging a bat again. Later that day, my entire rib cage tightened up and I had shooting pain throughout my collarbone area. Due to the coumadin, I couldn't take any Ibuprofin to reduce the muscle swelling so I was in pain that gradually went away in 5 days.
Moral of the story, no matter how better you feel, your body went through quite a shock during surgery and the muscles surrounding the sternum probably tightened up more than you realize. My advise is to stretch, take it slow and gradually increase your swing speed and repetitions. We are now into our second week of softball season, I've played four games and feel fine.