What Causes Screaming Of A 2 Year Old In Sleep?

Hello, My son is soon turning 2 and I suddenly see some changes in his sleep and food habits In the last week. he used to have a full bottle of milk before his sleep and would fall asleep while I calm him down by way of singing. it generally takes an hour to put him to sleep but he would lie down in the bed. since I am a working mom my mil takes care of him in the day and generally puts him to nap in jhoola. In the last 10 days my son has refused to switching off the lights (he screams as though he is frightened) and is refusing to drink milk(he consumes only when my Mil feeds him milk). There are no physical issues - we took him to his pediatrician as well and the only suggestion we have got from him is to get to put my son to sleep early and he is on pedichloryl 3 ml every night. I am surprised by this sudden change in behaviour and wanted to know what I must do to bring his old sleep habits again. even with pedichloryl he wakes up screaming 3 times in the night and would settle down sleeping only in jhoola...