Without delay, you MUST increase your intake of liquids, food, rest, and most especially the number of times you pump or breastfeed throughout the day. You must continue to eat for two if you are breastfeeding. Your body burns tons of calories to produce milk. Without food, no milk. If you're back to work, or on a post-baby work-out, you're probably burning extra calories, and that means no extra fuel to make milk. EAT. Don't worry about gaining weight. Breastfeeding burns alot, I mean alot of calories.
Most important, contrary to an earlier post, water does not decrease milk production. Like food, you need a lot of liquids, including and most especially water, to produce more milk. In some cases, drinking liquids while breastfeeding will help with let down. Drink more. If you prefer juice, drink juice. Your body can not produce milk if any of these above factors are lacking.
If you very serious about continuing breastfeeding, you're going to have to take some well intentioned measures and soon, or risk your body's total shut down of milk production. You need to jump start your body into thinking the baby needs more milk. Your body is a fine tuned machine that will only produce, what the baby needs. Even an artificial baby (pump) So if you only pump a couple of times a day, for just a few minutes a day, you may still produce milk, but eventually your body will stop producing enough to feed a child.
Before things slow down too far, you'll have to get radical and start pumping/feeding like you did shortly after the baby was born. That means, bringing your pump to work, and pumping both sides every two hours for 10 minutes on each side, for about a week to two weeks, until your body's prodction of milk increases significantly. Once you get production up to a level you find acceptible you'll need to maintain that pumping/feeding schedule until you are done breastfeeding for good. You can't stop. If you have a weak, non-hospital grade pump, this could be a factor too. Double cone pumps are the most time efficient. Medela is a good brand if you don't have one. Pricey but insurance covers most pumps, and there's used ones on Ebay. Just be sure to purchase fresh cones and tubing (get it at Target). Handpumps are alot of work, and often can't get the job done well enough to keep production up. You need powerful suction. You need to get those breast empty each time, to boost production.
This is burdensome for some, especially if you are totally depending on the pump to keep milk production up. One effective ways to cut down time on the machine is to breastfeed her at night, throughout the night. Those who exclusively breastfeed know a very important secret about milk production and nightfeeding in this manner. Stress can shut down milk production in a heartbeat. The benefit of nightfeeding is the child is feeding non-stop throughout the night, and at will while you are sleeping/resting. Many moms using this method have discovered their bodies produces some of the best milk, they produce more milk, and have been able to prolong breastfeeding longer.
There are many books and studies on this topic. You can read up on it at the La Leche League's official web site. Through the site you can contact a local consultant who can meet with one on one, or put you in contact with a support group in your area. Most LLL consultants are nurses/doulas and certified specialists that can help you one on one. Go to www.lalecheleague.org.
One last point, one other cause of little to no milk coming from a breast could be due to blocked ducts. If you have pain in that area, be sure to get checked by an OB. Mastitis is not fun, and the direct result of blocked ducts. You can find home remedies on the La Leche site if this is the case. Use these until you can get to your doctor. Good luck. HOpe this helps.