directly quoting from here http://www.breastfeeding.asn.au/bfinfo/lactose.html
something I wish I'd found soo much earlier as DS was textbook case. We still don't eat dairy just in case that was the prob but thinking of doing a few trial intros once this lot of teeth com in. He's 16mo now and we took about 12 mo to really sort the prob as poos would be green sometimes and not others and trawling the net just didn't help....
"Lactose overload in babies
Lactose overload can mimic lactose intolerance and is frequently mistaken for it. An overload is often seen in babies consuming large amounts of breastmilk, that is when their mothers have an oversupply. This may result in an unsettled baby with adequate to large weight gains. The baby usually passes urine more than 10 times a day and has many (often explosive) bowel motions in 24 hours. They may have green, frothy poos that resemble those of a baby with lactose intolerance. This usually occurs in babies under 3 months old. Ironically, a mother may thinkthat she has a low milk supply because her baby always seems to be hungry. The nappies can be the biggest clue to what's happening. What comes out the bottom must have gone in the top!
There is a vicious cycle here. A large-volume, low-fat feed goes through the baby so quickly that not all the lactose is digested (more fat would help slow it down). The lactose reaching the lower bowel draws extra water into the bowel and is fermented by the bacteria there, producing gas and acid stools. The acid stools often cause a nappy rash. Gas and fluid build-up cause tummy pain and the baby 'acts hungry' (wants to suck, is unsettled, draws up his legs, screams). Sucking is the best comfort he knows and also helps move the gas along the bowel. This tends to ease the pain temporarily and may result in wind and stool being passed. Since the baby indicates that he wants to suck at the breast, his mother, logically, feeds him again. Sometimes it is the only way to comfort him. Unfortunately another large feed on top of the earlier one hurries the system further and results in more gas and fluid accumulation. The milk seems almost literally to 'go in one end and out the other'.
Many mothers whose babies have had this problem have found it helpful to change from an 'on-demand' breastfeeding routine. This is usually only necessary for a short time. The aim is to slow the rate at which milk goes through the baby by feeding one breast per feed, or by 'block-feeding'. To block-feed, set a 4-hour time period (this may be adjusted according to the severity of the oversupply) and every time the baby wants to feed during this period, use the same breast. Then use the other breast for the next 4 hours, etc. Each time the baby returns to the already used breast, he gets a lower-volume, higher-fat feed that helps slow the system down. While block-feeding, check that the unused breast does not get overfull. When the baby's symptoms are relieved, the mother is able to go back to a normal breastfeeding routine and feed according to need."
hope that helps...