Wife keeps saying she's fat and she isn't

I didn't understand what she said so I said what baby? Then she says nevermind. So a bit later I asked her what she had sis and she told me. I replied with "babe you're not fat Stop saying that" then she says "see that's why I didn't want to tell you" so I'm confused... I was being honest when I told her she wasn't fat. She has a little belly but she's definitely not fat. I understand that she's insecure about her belly and she's working on losing it but she's not fat at all. I absolutely love her body regardless of what it looks like. What should I say to her when she says that she's fat?