Sad about breastfeeding!

My son will be 4 weeks old on Tuesday. He still hasn’t gained back to his birth weight, but the doctor said he’s gaining weight so he wanted me to keep breastfeeding. Well that was Monday. He’s been fussy all week and wanting to do nothing but eat. He’s not sleeping and he’s not having as many wet diapers. So I gave in and gave him a bottle of formula. He ate 3 oz( after nursing for an hour) and has been asleep for 3 hours! He seems so much happier now!