It's a good idea to try something one has previously eliminated from one's diet again, just to see if it was a one-off reaction or if one does have a genuine intolerance of the outlawed ingredient. Yesterday, only because I was being somewhat lazy and mean (I didn't want to open a whole carton of soya milk for less than a cup), I made a white sauce with part skimmed milk. This was for a vegetarian Moussaka and all the other ingredients I used in it are things I use regularly with no problem.
Once more, my stomach blew up into a huge ball and began to ache right after I'd eaten my dinner. In fact it ached so much that it ruined my entire evening: I felt too ill to do anything and all I could do was curl up in a ball with a microwave heated wheat bag on my stomach, groaning, until I fell asleep. I think I can now take it as a given that I am intolerant to dairy and, if I've been having this same reaction now for years, it probably wont change.