J.H.
My son is 15 months old, and like you it was always really easy getting my son down for bed. But within the past few months, at least 3 or 4 nights a week my son fights going to bed. He also runs around the house and likes to much on things (though we try and limit this before bedtime. The nights that are "good nights' are usually the nights that he goes to bed earlier then usual (around 7:30). We make his bottle, brush his teeth, turn off all the lights and shut the doors and drink our milk, then go to bed.
Maybe he is over tires and needs to go to bed a little earlier (try 15 minutes) sometimes that makes all the difference. Or whenever my son is crying in his room b/c he doesn't want to go to bed i go and give him a hug, explain to him that I will see him in the morning, and I rub his belly (usually by then his eyes are fluttering b/c he's so tired).
If he's screaming (i know some parents will disagree with me), go in his room and sit down will he's laying down. maybe all he needs is company (sometimes nighttime is a lonely time)
Hope it helps