S.H.
take her to the doctor, don't second guess it. Don't give cereal yet...she's too young, and at this age their digestive system is too immature and their internal organs are not fully developed yet.
Yes, a baby can be sick, even with no fever. Or sometimes there is fever and no other symptoms.
You just need to take her to the doctor.It is not only her appetite... she has other symptoms too. Tell your Doctor all of them. Also, as a baby develops their sleep patterns do change, especially at growth spurts and developmental changes where they get hungrier and need to feed more and are gaining alertness and in cognitive/physical skills. Sleep patterns are not static in babies...they will change.
A good book is "What To Expect The First Year" by Arlene Eisenberg. I referred to it all the time with my first baby.
All the best,
~Susan