My boyfriend has a close friend who is female which is fine...?

[Chillimohican]

I dont have a problem with the friendship as I have male friends too. I told him to invite her round for tea one night (I've met her a few times and shes friendly enough and we got along fine) so he sent her a text and invited her. Unfortunatly the evening had to be cancelled so he text to let her know. He said a few days later that he was a bit perplexed as she sent a text back saying ''I'll wait for my next invite like a good little girl'' and put a wink smiley face, so I asked what he'd said to her (apparently she has a new job as a teacher) and he'd put ''cant make tea on Wednesday as I have to work we'll have to sort another night when you can come round and tell me all about your new job, cant wait to hear all about it, do you get to tell me off now if I've been a naughty boy? heh heh''
I just felt a little unnerved by it as thats why it spurred her to put about being a 'good little girl' it seemed a little flirtatious to me, I have male friends but I wouldnt ever text them saying with the line 'good little girl' in it as I personally feel theres a certain line not to be crossed and they're my mates and I dont want to flirt with them, it leads to allsorts (previous experience with an ex sending flirty emails to girls and was cheated on) am I just totally over reacting? Sense tells me I need to pull myself together!!!! But just wanted your views on it. I've not said anything to him about it. Oh the two of them used to date a long time ago as well but that was a long time ago and I wasnt bothered by that but am wondering now if there's 'unfinished business'???