I'm guessing that as you'd stopped trying because it "wasn't happening" you'd resigned yourself to not having another baby? how long had you been trying?
I think that when you decide that something's going to happen/not happen, then if that changes it can throw you just a bit, because suddenly everything you thought you had planned for your life has changed.
We ttc for 13 months for our ds. Every month I was upset when af arrived, every time I took a pg test I just wanted it to be positive and it wasn't. And then after a year we decided that it obviously wasn't going to happen and we were going to have tests to find out what the problem was and then tell the family that there weren't going to be any grandchildren once we'd had the results. (I didn't feel that i could go through invasive treatments such as IVF etc).
So we had the tests. dh had a sperm test and when I started bleeding I went for a blood test. Two weeks later i rang for the results and was asked if I was "on the the pill". I just figured that my hormones were obviously so screwed that the test results showed that. That day I passed out at work, and because of the blood results I decided to do a pregnancy test - it was positive. And the anticlimax was tremendous. I sat in my bathroom shaking saying "but I can't be pregnant! I can't be!" (bleeding must have been implantation but it had been heavy so had never even thought it was anything other than a period. Of course I was excited in time, but the initial shock was huge, because I'd got used to the ida that we weren't going to have any children, and suddenly everything had changed.
You wanted this baby, you planned for this baby, it just hasn't come at the time you planned, but as things progress you will feel the excitement, once you hear that heartbeat, feel that baby moving, you will remember why you'd been trying all those months, and it will all be fine.
good luck xxx