31/2 mth baby wont stay asleep. She will fall asleep for 15 min & stay up till 5am.Very cranky. What do I do?

Since my baby hit the 3 month mark she changed her sleeping ways. She used to stay asleep all night long after I fed her. By now everything is diferent. I feed her and she falls deeply asleep on my lap. Even if I leave her on my lap her nap only last 15-20 minutes and she wakes up again VERY PERKY AND HAPPY TO SMILE AND PLAY. But then she gets cranky and starts crying and fusing. I know that she is sleepy but she wont fall asleep, she doesnt want milk or pacifier or to be held. Nothing works. Ive tried the swing, the bouncer, the sleep rutine, a book, a bath, cooing her, singing, swaddling. But she wont budge. Eventually at 5 am she will finally grab the and she falls asleep. HELP I work and I leave home at 6 am.