Please bear in mind that what you call "shivering" may actually be an episode of convulsion
Since your baby is already on phenobaribitone, it means the problems started soon after birth as she is only 20 days old.
It is very important to re calculate the dose of the drug after checking her weight. Also the dose has to be spaced at 12 hours. Also The divided dose is given in equal amounts and not 1 ml in morning and 2 ml at night. So give equal divided doses after you are absolutely sure of the weight of the baby.
Also it is likely that the dose might have to be increased if the convulsions are not under control. Do remember that EEG is not a modality of diagnosis.
My sincered advice to you is to consult a Neonatologist immediately and keep the baby under observation. Get blood electrolytes checked along with blood sugar.