What time is her nap / how many naps is she towing a day ? I would suggest starting by following the recommendations for wake windows for her age and see if that helps
I would just accept that she’s still young and sleeping patterns can constantly change. Could also be a growth spurt as well x
15 weeks is sooo young she is still figuring it all out. They eventually will get themselves into a routine and want to go to bed at a certain time. We didn't have a routine until our baby was 6 months old
Maybe wake her up earlier so that way you can put her to sleep earlier?