In general, constipating foods include cheese, banana, fortified cereal, rice, applesauce, and most bread products, especially those that aren't whole grain.
Foods that can help ease constipation are prunes, prune juice, orange juice, apples, apricots, and oatmeal. Increasing water intake helps quite a bit too.
These are just a few I can think of off the top of my head, I'm sure there are lots more and it often depends on your LO. Babies can get constipated suddenly if their food intake increases and they aren't getting enough water or other liquids. Also, babies poop less as they get older, so if your LO isn't straining or uncomfortable, your LO may just not be going as often. HTH!