Why is my 12 week son waking up in the middle of the night to eat when he stopped doing this at 8 weeks?

My son has been sleeping through the night since 8 weeks old. He would get up at 4:45am then he started sleeping longer and getting up at 5:30am and then he started sleeping until 6:30am. But now at 12 weeks, he has been getting up at 3:30am for a bottle. Why did he stop sleeping through the night. Also he is drinking breast milk not formula as I exclusively pump. He has acid reflux so he takes acid 3 times a day and has cereal in his bottle. He goes to bed full so I don't know why he is so hungry that he stopped sleeping through the night. Any suggestions?