Help!! Toddler not ready to sleep before 4 am 😭😭

my 18 month old for the past 1 month has been continuously pushing her bed timings.. Initially it was 12 and now for the past 10 days she is not ready to go to sleep before 4 am.. She wakes up around 1pm in the afternoon and this is driving me crazy... Plzzz suggest me how to bring her back into her previous schedule.. Will giving her Tylenol for 1-2 nights help?? Plz suggest