Its probably a plateau.
If your diet has been working fine up till now, then your body has become accustomed to the system and it needs a little push.
I recommend doubling your water uptake for a week. It will stop your body from retaining fluid and help to flush your stomach. You can also try a multivitamin or just vitamin c, both will remind your body that it's not starving to death.
Unless you are actually starving yourself, if that's the case, eat a big bowl of porridge (high fiber, low GI, high protein).
The other explanation is that your diet just isn't good enough.
try counting calories. eat half or even 1/3 of your calculated recommended daily intake. (half is healthier, but i do 1/3 coz i'm impatient).
But be persistent. Dieting, to me, is about applying constant dietary pressure to your system over extended periods of time. Time and pressure. It can literally move mountains.