Being under the prior impression it is not really a big deal, I think I would still like to moot. I am pretty sure I shouted it out, and then to make matters worse I swore because I thought I'd said it!! (I don't swear in everyday life at all) although I did giving birth both times 
but that's normal!
I can categorically state I was not thinking of my ex at the time (I have no sexual nostalgia for him now after 3 years apart), but having examined the instance I can only conclude that the passion was to a degree matching that of my previous experiences with my ex, in other words, most wonderful! Hence my outburst. He is my first lover since I left my ex.
My new partner is very sensitive about the issue and I am under the impression he is convinced I was thinking of my ex, despite my assurances otherwise.
I did try to explain to him that it was probably just the familiarity of the situation, habit, etc. I know I used to accidentally call incoming partner's by the outgoing's partner's name for some domestic trifle or other, ie "Want a cup of tea (insert the wrong name)?!" and it has happened to me in the past, to which I have just laughed or shrugged it off.
I realise however, that it is a potentially sensitive incident and need to know how to reassure my new man it was meaningless.
I was a little surprised he then discussed it with his own Ex, although apparently it just came up in conversation when she mentioned a similar episode herself, to which her response was first alarm and then later dismissal.
So. Is it possible I was subconsciously thinking of him? Because I was very much into my new man at that moment so I can't see how 
I just don't want to make the same error again, that's all.
Please assure me this happens to lots of women, or advise me on how better I may have handled the explanation?