Is the swing in motion while she is sleeping? If so she isn't getting into a deep enough sleep. When baby's nap in cars, strollers, or swings, it only produces a twilight sleep, and not a deep restful sleep. This is according to a leading sleep expert, Doctor Marc Weissbluth, author of "Healthy Sleep Habits, Happy Child."
My advice is also to put her in her crib, as others have said. Pack and plays are not comfortable. It is also best to let a child nap during the daytime, and sleep during the night time in the same place, so they associate a certain place with sleep.
Also, don't be afraid to let her cry for a while. It will wear her out, and while she may not fall asleep on her own crying, probably the minute you decide to pick her up, she will be falling asleep in your arms. By not letting her cry for 10-15 minutes, you are robbing her of a good nights sleep, because you aren't allowing her to learn to fall asleep unassisted, or learn to sleep in a new place i.e. her crib, in a horizontal position. Teaching a child to sleep well is an important life skill, like teaching a child to ride a bike. It might be scary and unfamiliar for both parties at first (yourself and your baby), but trust me, you will be so much happier if you hold strong, and allow her to adjust, even if it means a little bit of crying.
Good luck, I've been there!