Oops, sorry I didn't reply to this bit yesterday
I only figured it was worth trying lactose-free when I'd spent ages on the internet googling after effects of food poisoning (like you, I was starting to worry as it went on so long..) Anyway I read something about helicobacter pylori (HP for short) often flaring after food poisoning, and THAT can cause lactose intolerance, so I thought it was worth a try. My docs at that point had done the HP test but I didn't have the results yet. It was clear.. but as the lactose-free diet was helping so much, I stuck to it.
Just one thing I meant to mention, if you do decide to go lactose-free, and substitute dairy milk with soya milk - soya went through me like a steam train :( I think lots of people are ok on it, but I wasn't. Rice milk was fine though, and also the Lacto Free range of products that are in supermarkets.