what does this mean?

11.5 month old, 3.25/3.5/4. she is all of the sudden not taking her second nap, she will be SUPER sleepy while nursing and when i lay her down in her crib seems like she will go to sleep. and minutes later she will start sitting up and trying to stand in her crib (also rubbing her eyes sometimes). is she not tired enough?? i’m not sure what to do