Unless your baby is exclusively formula-fed, it could be something that you're eating - dairy and gluten are common culprits.
I think what these other mommas have said is right on. I would add, though
-if you are breastfeeding, it's possible that your babe is getting too much fore-milk. In case you're unaware, fore-milk is fairly thin, like skim milk. Hind-milk is thicker, richer, almost like cream.Is your baby very gassy at all? I don't remember all the other symptoms of too much fore-milk other than green poo and gassiness beyond the norm. The solution for my two kiddos was (#1) to offer only one breast per feeding. When that didn't seem to completely take care of it, I switched to (#2) a 4-hour routine. No matter how many times my babes nursed during that 4 hours, they got the same side. That did it for us. They still got a little fore-milk with the additional feedings, but they got a lot more hind-milk with either #1 or #2.
If you'd like more experienced input or advice on bf-ing, you're local LaLeche leader can help. They do phone consults (free), so you wouldn't have to go anywhere if you didn't want to do so.
http://www.lllusa.org/web/PlanoTX.html
I hope this has helped some
K. H, mama to
Catherine Anne, 4yrs (nursed for 2.5yrs) and
Samuel Anthony, 1yr (and still going strong)