OK I know a bit about this. Lactose intolerance is on the rise, it isn't really unusual/rare. It is often missed / confused with other things. it isn't an allergy
My DS was diagnosed with Lactose intolerance at 6-7 months after weeks of horrendous nappies. at one stage we were getting 10-12 dirty nappies a day. For a long time we were told 'it's a virus' and it can last weeks (sometimes over 2 weeks for virus to get out of baby's system)
We had to keep going back to GP with me saying 'this isn't getting better' and it's now been x weeks. Luckily GP listened, and prescribed special formula - Nutramigen (not one you buy off the shelf). We were told that we would know v quickly if lactose was an issue and indeed in 3-7 days the nappy problem resolved itself and we were back to 3 dirty nappies a day at most.
I panicked and stopped breastfeeding as I was told to cut dairy out of my diet if I was breastfeeding and it seemed impossible being vegetarian (no one told me there were good dairy alternatives easily sourced in the supermarket)
Anyway, looking back I think DS had signs of lactose intolerance that were missed by HV and others when he was much younger, i.e. lots of wind, not sleeping much, feeding little and often, stomach cramps, etc
Good luck, speak to your GP and it's not the end of the world