Playing or fighting???

So I have two staffys, Tyson who is a 7 year old male, and Lacey, our 6 month old girl. They get along so well (luckily Tyson is very tolerant) most of the time. Yesterday they were play fighting which they do a lot and are usually very rough about it and make lots of noises which I'm used to now. They were going along fine and then I noticed that Lacey's fur on her back was completely standing up which I have been told is a sign of distress/aggression. All of the other signs seemed as though they were still playing but I decided to separate them for a while. A few hours later I re-introduced them and they were back to normal. They were playing again but not as full on as normal. Then all of a sudden Tyson snapped at Lacey and they got into a serious fight. Neither of them backed down so luckily we were able to separate them before anything got too out of hand. I understand that dogs have fights and can't be best friends all the time but it really shook me up, and It's been on my mind ever since. I'm wondering if I should have separated them for longer when I saw Lacey's back up in the morning? Or should I have seen some other sign of tension? Would love some opinions. Thank you!

My 2 have had some full on scraps , normally these will be triggered by food , jealousy or toy posession , ring any bells ??

They've been really good for the last few months so we have now taken the ultimate step not to separate them when we go out , normally we come home (only after 2 or 3 hours ) to find Tillly curled up on my computer chair and Tommy on his bed in the hall.

Just like mates , brothers and sisters you will experience fall outs from time to time. Most important is that both of them learn the boundaries ( sounds like Tyson was setting his) and they have room to escape . So for example if they both have the run of the house then great they can escape each other and have their own time out , if not then think about assigning areas to the house, do not put them both in a small room ( kitchen etc together ) where they are both facing each other off

Mine do that from time to time. There is even blood sometimes (nothing too serious just scrapes). You do need to separate them but when I get in there and toss them apart (once it literally took a tossing apart) they seem to snap out of it and even be shocked themselves. Kinda like two teenage boys... Just keep your eyes and ears open and you will know when it is going past play. They move on, but this is one reason I don't leave them together when I go out.

Sorry I should have added that we did keep ours separated until a few days / weeks ago based on good behaviour and their relationship . If you have any doubts at all then always separate them when not supervised