What I understand from reading previous posts and info on web, this is apparently a common problem among 3-4 month old babies.
My son now 4.5 months old, also reject milk when he's around 3+ month. Was very worried coz he was rejecting my breasts also.
What I suspect for my baby, but not sure if applicable to you since your baby is on formula.
- he just had his 3-month injection so not feeling good thus affect appetite
- he got blocked nose
- he was actually not hungry i.e. can last longer between feeds. Used to feed him every 2.5-3hrs, then progress to 3-4hrs, now sometimes even 4-5hrs! (I'm latching him so also dont't know how much he drinks each feed - that makes me worry more)
- he was very distractable (still is now!!), too interested to look around and play and forgot about hunger. Read from somewhere that during 1-3 months, babies basic instinct is survival and contented to just drink milk. From 3 months onwards, they become very curious and wants to explore. Feeding becomes secondary. They dont't know how to balance both needs. I'm also wondering when this distractable phase will last... feeding my baby is a big challenge every day now, got to make him a little drowsy first otherwise he will just drink a few minutes then stop and play, or pull in and out.
- he's a bit sick of milk (that's according to my mother in law)
Till now I still get stressed up when he doesn't want to drink. But keep reminding myself to trust the baby and dont't let daily intake of milk affect me. Look at long term. So long as he's pooing and peeing and gaining/maintaining weight (I weigh him weekly), it's ok. My pediatrician also says so long as double birth weight by 3 months and gaining weight steadily weekly, it's ok. I think this phase will pass. Apparently after 3 months, babies pattern keep changing!!
Hope my above experience help. But if situation worsens, better go see pediatrician.