I am kind of dealing with this too. Had a tricky time with DD's birth and her conception (I have severe endometriosis and was advised children would be impossible, though DD was totally natural to the shock of my Drs!) so we decided fairly early on that we'd "beaten the system" by having her so wouldn't try for a 2nd.
Have been fine with this, but now DD is 4 and due to start school in September I'm wavering..... I think it's because a lot of the Mums at DD's nursery are younger so are on No 2 (or 3. Or 4!) and I do look at their tiny babies and wonder if we're doing the right thing. Also, I'm a sahm - largely by choice but also due to my health so am wondering what the hell I'm going to do with my days when she's out all day, every day!
DD seems happy and secure and very outgoing, and I now wonder how she would react to a potential sibling after it being the 3 of us for so long. Plus we're lucky that, as we only have DD to worry about, we can make decent financial provision for her future whereas with 2 money would be tighter than it is now. Now she's older she loves having adventures with me and DH and is a lot more "portable" than a baby, plus my DM has her once a month so we get a chance to do some grown up things - I'm not sure how that would work with a newborn / baby in the mix.
I think my recent pangs of something are linked with the transition to school and me being 40 and my endometriosis symptoms returning, more than the real heartache of yearning for a baby I had prior to DD's arrival.
It is a tricky one though, as I think it's considered quite controversial still to "only" have one - even my MiL, who knows how hard it was conceiving DD, still goes on about When we have No 2..... Sigh.