Alex never had lactose inrolerance until after the first time he had gastro.
I would take Lucie to the docotrs to get her checked out they should get a poo sample from Lucie to just check it's still not gastro.
Also check that she has no while marks in her mouth because after the last lot of gastro Alex caught oral thrush and it's was just the same as him having gastro but this time he was throwing up with it. That came on afterr about 3 days of gastro stopping as his immune system was low.

I hope you can find out what is wrong with Lucie soon.