Lactose intolerance is quite common, and can certainly develop at any time. In fact, as you get older, the body stops producing quite as much lactase, the digestive enzyme needed to digest lactose (milk sugar). Speaking of which - there are two types of dairy intolerance: casein intolerance (milk protein) and lactose intolerance (milk sugar). More common is lactose intolerance, and the good news is that there are some dairy products that do not contain all that much lactose. In general, lower fat dairy foods contain more lactose than their full-fat counterparts. For instance, something like skim milk is going to contain *more* lactose than real cream. Also - hard cheese contains very little to no lactose, where as something like cream cheese or cottage cheese will have more lactose.
I would strongly encourage a month of 100% dairy-free, and then after that - try eating the lowest lactose dairy foods, one at a time, until you can determine whether that food bothers you or not. By the way - butter is the easiest to add back in. After that, it'd be something like real, aged parmesan cheese, or some sharp cheddar etc.
Meanwhile, I used to have a problem with lactose, and I went dairy-free for a good long while. Weird thing? It turns out that my real problem was gluten (wheat, oats, bran, barley, rye), and the gluten problem had damaged the villi in my intestines - and it's the very tips of these so-called "villi" in our intestines where the body produces/makes lactase (ie. the digestive enzyme that allows us to eat dairy foods with lots of lactose - a.k.a. milk sugar). After I went gluten-free, my seeming dairy intolerance cleared up quite a bit (I'm guessing the villi healed up?), although I still don't eat a lot of dairy foods in general. A bit of butter here and there, and the occasional treat of some fancy cheese or what have you.
After say a month of completely dairy free, try some butter. A week later, try some hard cheese. For things like milk - you might want to keep on with dairy-free alternatives such as Blue Diamond almond milk, or coconut milk (my favourite brand is Native Forest). I drink coffee and tea, and full-fat coconut milk is better in coffee (creamier), whereas the thinner almond milk is nicer in tea. Beware that these are both an acquired taste though - you can still taste a hint of almond in the almond milk and same with coconut milk. By the way - the almond milk is very low-cal, low-carb and quite good, imo. In the summer, I make shakes with the unsweetened vanilla almond milk and whirl it in the blender with a bunch of frozen berries. Delish! Don't despair - there are things like coconut milk ice cream, coconut milk based yogurts and the like.
I tried the Lactaid type capsules but in the end, I decided it wasn't worth the hassle and they didn't alleviate my cramps well enough to bother. So, I went dairy-free for a long, long time.
If you like - maybe give us an idea of which dairy foods you're currently eating, and I bet that we'll be able to help you find dairy-free alternatives to help you get started on a dairy-free trial just until you feel better and can suss out what's going on (ie. is it lactose or casein or something else?).
Hang in there - I know it's no fun, but it is very common, so you're not alone. There are tons of great substitutes out there too, so son't despair!
Take care, and hope you feel better soon.
PS - plain peppermint tea is amazing for nixing the bloat/cramps ...give it a whirl, it'll help.
PPS - the lower fat dairy foods are the *highest* in lactose content - full-fat cream is lower in lactose than skim milk. Later on, as an experiment, you could potentially consider trying real cream diluted with water for your tea instead of low-fat milk. Also, I don't think butter really has much lactose at all, and as I mentioned, hard cheese has very little lactose. Hope this helps a wee bit.
