What Causes Increase In Size Of Stomach Area After Delivery?

I look like I am 6 months pregnant, I had a baby over a year ago and this is my 5th baby. My doctor says it is just fat. I am slim everywhere else. I had gestational diabetes in 2 pregnancies but I have tested clear afterwards. I put on new trousers today and they fitted my waist, now at nightime my trousers are cutting into my waist.I am trying to diet but I feel that I may lose weight but I won't lose the stomach. It is there constantly.I have had a scan of my ovaries and cholesteral and sugar blood checks, all are clear.Do you have any idea why it is this way? I am in my early 40's.