It might be their actual protein, I know a lot of guys dont have issues with it, a lot of guys dont have issues with the powernutrients protein either, but I did. I had major issues with their protein, Id take it Id get massive cramps and be heading for the toilet with in 3 hrs. I went off protein for a while, then tried Aussie bodies and Im fine with their protein, Im also okay with designer physique but have issues with a few others. I would give it a rest for a week or two and then try designer physique wpi.
If you end up trying to use lactase, use the drops not the tablets, the drops convert lactose before you drink it, where as lactase tablets convert it in you, there is a chance that taking the tablets will down regulate your bodies own production of lactase (however small it is) and possibly make it worse.
Dont try rice protein, I tried that and I threw it out, it tastes like what I imagine shit covered in dirt would taste like. It would just be a waste of your money, at least thats what I think.