>has played happily with our two children, play biting, wanting lots of tummy tickles<
It sounds like the play biting has got out of hand, a stop should have been put to this sometime ago.
I would suggest no more play with your daughter until she stops 'play biting', as soon as it starts, turn your back and walk away. Your daughter should be training her to do simple things like sits and down, feeding her, making her sit and wait prior putting her food bowl down.
Don't worry too much, at around 5 months they can go through a stroppy stage. Just be firm and consistant with her, any sign of biting, play or otherwise, she does not get your attention.
Sandra