How do you know you stopped producing milk? The 6 month growth spurt is a beyotch. If your clue was anything but a drop in wets or poops, then your production may have been fine. At this time, baby's demeanor can get pretty pissy, clingy, and colicky. We just did the 3 month growth spurt and I kind of feel like my nipples are gonna fall off. LOL However, I have been through a drop in supply. My daughter dropped down to 1 wet/day when my supply died due to pregnancy.
There are quite a few reasons this can happen...from stress, not enough pumping (or nursing), birth control pills/IUDs, antidepressants, dehydration, and, sometimes but rarely, just fluke. Consider anything that you were taking from about 2 months of age on.
One huge culprit I've found that most women disregard due to manufacturer information is the Mirena IUD. The Mirena claims to have no effect on breastfeeding, as do many low-estrogen birth control pills. In fact, I've counselled several lactating women who had supply drops within 2 months or so of being implanted that wouldn't recover. Some take out the Mirena and recover within a month or so.
Dehydration is another big culprit. I have, personally, found it is more difficult to nurse in the summer time due to increased fluid demand by my body. Also, I've seen more supplies die in the summer time. You really have to pump about 80 oz of water (non-caffeinated and non-carbonated) into your body to have enough for a 4-6 month old. Consider they are drinking about 30-36 oz of breastmilk. Our bodies are recommended 6 glasses a day (64oz). If you just drink that much, then your body is only getting about 30 oz a day (with the rest going to baby). The body starts to leach that from the milk supply over time, I think. Drinking water alone can be a full-time job when breastfeeding!
Otherwise, thyroid dysfunction and other medical problems can lead to drops in milk supply. You really have to research what may have been going on in the months prior to the drop.