What makes you think your supply is low? Is your baby not gaining?
I ask because from what I've read, weight loss would really only affect supply if you were already underweight and malnourished. Stress can inhibit the letdown reflex but exhaustion is pretty much par for the course for all new moms, breastfeeding or not.
Many women THINK their supply is low at about four months because their supply has regulated and they don't feel full and leak anymore. As long as your baby is gaining weight well - kellymom.com has some great resources on normal growth - all is well.
The best way to increase supply is to nurse, nurse, nurse. Offer the breast more often, offer the second breast after baby has finished the first, use breast compressions and co-sleep. A big bowl of oatmeal every day for breakfast is worth a try, too!
If that doesn't work to increase supply, pumping and herbal and/or prescription galactagogues may help.
Megan, loving her sweet boys, born Aug. 2008 and Feb. 2011, and their sister, born still March 2007