holding breath

i left my 6 month old DD at the gym childminding service for about an hour. when i went to pick her up she had been crying for 15 or so minutes she was really tired and a little overdue for her sleep. when i put her in the car seat to go home she was crying alot and then held her breath for AGES!!! it was like when they hurth themselves and cry and then there is a long pause before the next cry sound but it lasted for a long time and she wasn't hurt in any way. i quickly pulled her out of the car seat she started to cry and make sound again (thank god) but her mouth and lips had turned blue from holding her breath!! why would she get so worked up and do this? do you think she did it on purpose? i dont understand why it would have occured. i hope it never happens again it scared the hell out of me!