What is this? Pls help

It could be either. Try not using the Aveeno lotion for a couple of days and see if it clears if not then possibly milk allergy. You could try switching your fromula to a lactose free one (which you can purchase from many chemists). Process of elimination and see what works if neither then time to see your GP. XX

Does it feel hot? Are they teething as it can make their cheeks go pretty red tbh, have they had injections very recently? Xx

I am breastfeeding so dnt know what happened

possible hot coz I am not in UK rn, teething i dnt know he keeps itching his lower tooth though