Of course you're not crazy! I weaned my daughter at about 18 months. It was the right time for both of us. But because she had been on whole milk for some time, I wasn't working at the time, and she weaned slowly, I had given up pumping several months earlier. And as you expected you would feel, I was joyous to give it up. But there were two big differences between my situation and yours. First, when I stopped pumping she was still nursing. And second, she weaned at a time that was mutually desirable for both of us. I think if you had stopped pumping while still nursing, you would be mourning the loss of nursing more and not the pumping. I suspect this sadness you feel is more tied to the fact that in some way, subconsciously, you were still associating the pumping with nursing. So you probably didn't fully work through your emotions associated with his weaning. And now you are forced to face that reality. For a mom that enjoys nursing, or nurses for an extended period of time, it can be difficult to let go of. Breastfeeding is one thing that, literally, no one else in the whole world can give to your child. So whether it is a bonding time or simply utilitarian, it is very natural to feel a strong sense of loss when that period in your child's life ends. And yes, your emotions are likely compounded by the expectation that he will be your last child. I still plan to have another someday (hopefully!). But many of my friends who have "finished" having children say it is harder to give up breastfeeding with the last one.
It's hard as a mom when unexpected emotions creep up on us. We have so many things on our plate to deal with that we don't leave room for the unexpected and we often don't give ourselves permission to work through them. Your relief will come with time, perhaps much more slowly than you anticipated. I am sorry you feel like you are missing out on that celebratory, elated feeling of being done though. But then again, you may be surprised again and feel some of it when you actually put your pump away for the last time. If you are like me, you may tend to mourn losses when you anticipate them, making the actual loss somewhat easier when it finally comes.
Cut yourself some slack. Your emotional state may have surprised you, but as you can see from all the responses, it really isn't that surprising! Hang in there and try to focus on the new, fun aspects of your son's development.