I'm Three Weeks Out From An Appendectomy, My First Surgery,

I'm three weeks out from an appendectomy, my first surgery, at age 31. I had constipation the first day or so after the operation, but after that it seemed to go back to normal. I didn't exactly eat great after that; lots of fast food and probably not a lot of fiber. Eventually, this past Wednesday I started having a bit of pain in my right side and I wasn't having a bowel movement. On Friday, I had very sharp pain and felt terrible. When I got home I took 3 tbsp. of Milk of Magnesia with a full bottle of water, but I still didn't have anything until the next morning, at which point I barely passed anything. I did feel a bit better afterwards, though. No fever, chills, change in appetite, or anything else; just the constipation and occasional sharp pain if I eat too much. Is it just constipation, or could it be more serious? Should I see my surgeon?