Perhaps, like me, you are having trouble getting the mother and lover thing straight in your head.
I've recently realised that this is (or was) my problem, although it goes a little deeper as I was sexually abused by my father (spit) from about 3 - 11 (ish), so I guess on some fairly deep subconscious level I equated sex with as a parent as dirty and inappropriate. This is weird as DH has 2 kids (DS1 & DS2) from a previous relationship, so he has always been a father for as long as I have known him. I think the fact that we had a child together (DD, now 23 months) set something off in my head which put up a subconscious sexual barrier. I'm sure a shrink would have a field day with me, the woman who married a man and a father 14 years older than herself !
Obviously, mine is a much more complex situation than yours (I hope) so not all I have said applies to you, but I do think that perhaps you are having some problems with sexuality as a parent. I think, for me, realising where my problems stem from has been the first step on the road to recovering my marriage and sexual relationship with DH. Perhaps if I'd have had 'help' from a pro, I would have come closer to discovering the problem much sooner.
A sex therapist may be a good route, but be aware that if you are planning to go the NHS route you may have quite a significant wait. Perhaps you could start with finding someone you trust to have DS for the evening and go out for a 'date' with DH ?
I know it sounds obvious but I sat down and told DH why I felt the way I did (he already knew about my childhood), it made him much more understanding of the problem. I think it made him realise that I wasn't witholding sex for power or a feeling of control or even just be spiteful, it was an altogether more confusing reason.
Fio is right, it could also be down to a hormonal imbalance.
Sorry for the epic ramble, I hope you can dig something useful out of it.
NdP
xxx