Possible Allergy

Long shot - please bear with me. So, as a tiny baby, M had severe baby acne and colic. After giving up dairy/soya in my diet, this slowly eased - took about 6 weeks. Then reintroduction had no impact. Weaning - totally fine. However, he has had what I thought was teething rash, which came and went for around 6 months. I now know this was eczema. At about 14 months, the rash started getting really bad, lots of back and forth with gp and multiple creams/emollients/steroids. Some helped short term then the rash came back, sometimes even worse. After having macaroni cheese one night (had been a favourite for months, so def not the first time) he got the worse flare up I’ve seen, so I stopped dairy for him ( not me, still breastfeeding). 3 weeks later, his rash almost completely disappears. 24 hours after reintroduction, he has a tiny rash (not anywhere near as bad as it had been) but also a bit of diarrhoea and mucus poop. Up until this, his skin is the clearest it’s been in months. Am I looking too much into this? Can a diary allergy get worse over time? How can we confirm it’s this and not a fluke? We are on holiday so lots of things different. Had anyone else had a slow increase in eczema due to build of of allergy? Thanks 🥰🥰🥰