increased sleeping

My daughter will be 4 months old in 2 days. Last night was the first night she has slept 7hrs straight. She woke up 2hrs ago only to eat 3oz of formula and went back to sleep. Is this a growth spurt or something else? I know i should be thankful but i cant help but to worry. As she has never done this before. Any ideas? Or any other babies did this around 4 months? She was born a month and a half early.