Well, I would definitely go and see your doctor, as I was in the same vote like you, I finished nursing my second and I just knew I was pregnant with my 3rd.
Every one is different but it had happened with me.
In my case I went to the doctors two weeks after, he wanted to put me on the pill, I said no I would like a pregnancy test first, he doubted that I was pregnant...
I went to see another doctor and sure enough I was right. I was told that nursing alone for some women is not enough and she advises woman to take the mini pill while nursing.
So any thing is possible, seek medical advice...
wish you well