Alot of children seem to go through this stage. I know both mine seem to be up and down with their appetites. The way I look at it is, I've never heard of a toddler starving themselves and I appraoch both my children with the attitude that they will eat when they're hungry-which is actually what us human beings should do. When my two are on a low appetite phase I try to give them lots of healthy snacks, so that they eat little and often, rather than trying to get them to eat large meals three times a day. The other thing I find they enjoy is drinking smoothies, which you can pack with fruit without them really knowing they're eating/drinking something nutritious.
It can be very frustrating, but just keeping in mind that they will eat when they're hungry really helped me get through these phases.
I hope that helps
Jools