3 month old fighting naps

My 15 week old is suddenly having problems with taking naps now, before she would take her pacifier and fall straight to sleep. Over the last two days she refuses her pacifier as she prefers her thumb. But even that is not working for her. I've tried putting her down for a nap before she gets overtired , but eventually I have to cave in and nurse her to sleep. I do not want to create a bad habit so is there anything that you have tried that has worked.