Softball Sized Lump in Abdomen

I'm 31 years old and have had 3 c-sections (1999, 2002), the last one was May 2006. About a year ago I noticed a lump (golf ball sized) on the right side of my abdomen. This is off to the side, below my belly button but above my incision scar. Over the past year the lump has grown in size so that my abdomen is lopsided (it's now about softball size). I've noticed that monthly, about a 5-7 days before my period, I am getting horrible back and hip pain. I'm literally unable to get out of bed sometimes. Occasionally this is coupled with a burning sensation internally running from my bellybutton straight down to my plevic bone. I originally assumed that this was normal scar tissue from the surgeries but now that I've noticed it's grown so much, I'm getting more concerned. I've scheduled an appointment with my doctor but am wondering if there are specific questions I should be asking my doctor so that this isn't just brushed off as fat or unimportant? Please note, when I lie down on my back, the lump is not really noticeable... it's when I'm standing up and I flex my ab muscles that this very solid ball is formed. Can you please give me some ideas of what this might be... as a single mother, I'm scared that this might be more serious than I originally thought.