Absolutely! This is one of those topics filed under "Things no one wants to discuss about breastfeeding." There are so many different factors at play. First and foremost, it's a very dramatic change in your relationship with your baby. Ideally, you were ready to wean, but even if you weren't and this was more of an abrupt end, it may signify to you the end of your special time with your baby. But this is far from the truth, as plain ol' cuddling never gets old and there are many other ways to maintain that closeness. Everyone has to do it at some point, and whether you weaned at 1 month or 1 year, it's very normal to feel emotional about it.
In addition, we can't forget about hormones! They play a significant role in weaning. When you stop breastfeeding, there can be a sudden shift in hormone levels that cause you to feel sad, down, depressed. Many women equate the feeling to having horrible premenstrual syndrome and that's OK as well. Once again, this is very normal. However, if your depression does not seem to get any better within an adequate period of time after you have weaned, you should speak with a mental health professional to rule out any other issues.