Clairey - not sure if this is similar to what your dr was talking about but a few years ago when I first moved overseas on my own, I had a real stress/anxiety problem and it all came out in physical symptoms. I often felt giddy and faint, I couldn't breath properly and almost felt like I had to yawn to get enough oxygen and I had a tight chest. Saw the dr, who diagnosed anxiety and suggested relaxation techniques. Anyway, just knowing what the problem was really helped, then I started yoga and pilates classes which did wonders. Soon after I got together with my now dh and life just got easier so gradually as I got less stressed, the physical symptoms disappeared.
They have never come back in the same way but I still get different physical symptoms when I am under stress.
Skid - like the term "nike" sex, it's SO true.
Clairey - perhaps I shouldn't do this on mumsnet but could I put in a supportive word for your dh? I kind of see where he is coming from about the third child. It sounds like you have a lovely little family and perhaps from his point of view he thinks another child is just going to push you further away from him. I think having a 2nd child in particular is hellish for a marriage, I have to say in our case it has been much harder after the 2nd than 1st. Having a third will presumably mean you will have just that bit much less of you for him (IYKWIM).
Instead of thinking you'll never have a third perhaps the way to deal with it is just put it off for now. If I was ever to do it again I would have the third when the 2nd was at school, so I could actually enjoy it properly. I realise there are arguements for having them close together but physically and mentally I think personally I just couldn't hack it.
And you never know if you leave it a few years perhaps when the time eventually comes you actually won't feel the same urge as you do now.
Ok tell me to go away and mind my own business now!