My boy was like that too when he was 0-2 mths old. It appears to me that he's self soothing to sleep. Sometimes they wake from a sleep cycle n to continue sleeping, they need to self sooth etc. This also happened during the times he woke for night feeds. He would do that after milk when i put him back in his cot. After a few minutes he fell asleep himself. As long as he isn't crying, I wouldn't worry.