Stop it!

A curious thing happened last night, which hasn't happened before... after going down on Mistress R and making her cum, she got me hard and then mounted me. Despite having just cum she was still extremely tight and I struggled not to cum inside her without permission, eventually I managed to keep myself under control (though only just it must be said) and Mistress ground herself against me. It was a little bit frustrating actually since it was more of a grinding motion than an in-and-out motion (for want of a better description) and I eventually started trying to initiate more of an in-and-out motion myself.
After a short while Mistress R simply said 'Stop it!'. It rather caught me by surprise a) that she said it, and b) the way she said it.
In truth Mistress R isn't prone to overly dominant behaviour outside our sessions, and this was a welcome development I thought... and quite significant really, because though she demonstrates her dominance by slapping my cock and balls etc, that's something she knows I love and want, whereas displaying verbal dominance is always a little harder I'm sure, especially the way it was phrased.
It was frustrating being told not to move the way I was when I was trying to make it better (I thought for both of us, but on reflection I'm pretty sure she was enjoying it the way she was making it happen...), but then I could tell that Mistress R was very satisfied with the desperation I was showing when she climbed off and I begged her to touch my cock for just a little longer (which she did, briefly).
Slowly but surely we are still progressing I think...