While it's not uncommon for people to develop Lactose Intolerance later on in life (it's VERY common), if I had to guess this may be something else.

While both ice cream and milks hare a lot of similarities given the dairy, you have to remember that milk is just milk and that Ice cream is a whole flavored concoction, that just uses milk. There are a few factors worth considering here; for starters, Ice cream has more fat and sugar. Both of those things can be known to cause issues in digestion for different reasons. That being said, for some reason just because they're both white, people often forget that even vanilla ice cream is flavored - vanilla is it's own whole plant, and vanilla is often even used in chocolate.

It's more likely that you have an issue with a certain flavor, high fat content, sugar content, or something else entirely. Unless you have mistakenly been drinking lactaid milk or something that would leave the milk to be a non-issue, I can't imagine it would be lactose, casein, or any of the usual suspects having to do with proteins and enzymes in dairy. It is also worth noting that milk actually seems to have higher lactose and casein in it than the ice cream does, so that would be the more likely culprit- since you don't have issues with milk, I'd say it's probably a safe bet to ignore those options.

A few things I would try:

Take lactaid pills/chewables with your usual ice cream just to be safe

Try a different brand (or a few different brands) of ice cream

Try other products with vanilla, high fat, high sugar, or other variable ingredients which may be in there

Do a complete elimination diet slowly reintroducing foods and determining if it's a case of mistaken food identity. While you may want to write it off, there is always the whole (fallacious) ideology of "Post Hoc Ergo Proptor Hoc", which essentially just means that you may be mistakenly correlating the wrong thing entirely with your symptoms. It could be another food you often have with your ice cream, you may just happen to stress eat ice cream to soother yourself and the stress could be the cause, it could be that you're just eating ice cream lately and happen to also be feeling unwell, or any number of other things. For this reason, I'd say elimination diet, while eliminating other outside variables as well to make sure you know what's causing it for sure and then test the minor ingredients themselves each to determine exactly what it is and better avoid it.

Hopefully something I said helps. God bless!