@fify-- It could be worms too. Pinworm (or threadworm) infections cause itching buttocks. Especially if the itching is close to the butt crack, you should consider this as a possible cause as well.
My son had a pinworm infection when he was seven and constantly complained of his buttocks itching. We put a piece of scotch tape near the crack before he slept to check for worms. Pinworms are know for moving outside at night. A couple got stuck on the tape and we saw them in the morning. It sounds gross, but this can be a good way to check.
Even if you decide not to try this test, please take your baby to the doctor and have him or her tested for worms. Pinworm infections are very common among kids and babies.