Raised hackles when playing?

I'm not sure if this is something I should be worried about or if it's normal behaviour for 2 dogs playing.
Bella is 5 months old now and the last couple of weeks when she is playing rough with Millie her hair starts to raise up from her neck right down to her tail. When its happened in the house she ends up throwing herself onto her back with Millie standing over her, sometimes with a paw on her head. Is it just over excitement or should I be worried that its aggression? would that start to show at such a young age?
At home when it happens, after Millie has done the paw thing she goes and picks up a toy and drops it in front of Bella.

You have to watch when they play because it can get out of hand and turn in to the real thing ive seen it happen a few times, its best to nip it the bud when they start...

Hard to say without a video but here is a link about body language

http://staffy-bull-terrier.co.uk/dogbodylanguage.html

I've stopped their play a few times when it's seemed to be getting a bit too much but I wasn't sure if this new hackles raised thing was something I should worry about or not and she suddenly goes deaf when it happens. From the link about body language It's maybe just Bella getting over excited and not necessarily aggression? I will stop it as soon as I see it happening though.
Thank you

It sounds to me like Millie is reinforcing her position in the home but I think that Bella may be starting to challenge this. I think you might need to nip this in the bud by asserting yourself as top dog so to speak.

You should always know your own dog this will come by watching them my girl is all growl and hair up all the signs but wouldnt hurt a fly so would say she was excited .
My other shows nothing, but wouldnt back down if challenged hes always ready.
Experience with them will tell you when to stop play and like the others say it soon can get out of hand

Thank you everyone. I'm still learning about how 2 dogs interact with each other, especially 2 living together and it just worried me that it might be a sign she was going to be aggressive, not that it would make any difference to how much I love my furry baby
I'm starting to understand how my poor mum felt when me and my brother would start

haha.

caroline, that last comment of your 'now i know how my mum felt when me and my brother started' make me giggle.

x