waking up in the night

My son who is 3.5 months old woke up in the middle of the night hungry tonight..he did a few night ago as well..He's a great sleeper at night usually. He had about 5 ounces of formula before bed which is what he normally takes. He usually doesn't wake up until at least 6 hours. Tonight it was just under 4 hours. Anyone else's baby doing this suddenly?