The 32-year-old has been paid four million dollars by Weight Watchers to lose the weight she put on after giving birth to her first child Maxwell in May and promote their brand.
Although she has dropped 40 pounds in four months, her cruel detractors have hit out at her for not losing the remaining 40 already, reported In Touch magazine.
"For years, Jessica has been ok with her body and everyone loved her for that. But now she's not feeling the love and she doesn't like it.
"She's trying to conceal her hurt feelings with humour right now. She uses jokes and goofiness to hide the fact that she's really sensitive," a source said.
Simpson unveiled her weight loss on TV last week and friends say she worked incredibly hard since giving birth to lose the extra weight.
"The past few weeks were really difficult because the pressure was enormous. She knew everyone would be judging her. She was proud of her progress so to face all these cruel taunts now, it stings," the source added.