28 Weeks pregnant and no baby bump

I have always been a overweight person. I am 28 weeks pregnant and I do not have a baby bump. All I have is my usual jelly rolls. Its frustrating to see other pregnant women rub their belly's with smiles on their faces. No one can even tell I am pregnant. My doctor says that my baby is a perfect size and is growing nicely. Why do I not have a baby bump?