Another sufferer here. As Cartooniverse pointed out, there’s really no way you can take a lactaid pill after the fact. The trouble is basically we are missing the enzyme lactase, which breaks down the milk sugar lactose. So those helpful little bacteria in our intestines will happily do it for us, resulting in symptoms with which you are probably all too familiar.
There are chewable pills on the market, which should be taken just before consumption. There are also tiny bottles of drops which you can add to larger containers of stuff like milk or yogurt. The drops need to be added a day in advance; they contain the enzyme which breaks down the milk sugar prior to your consuming the dairy product in question. Because the milk sugar is already broken down before you eat it, the product you added the drops to tastes sweeter. You may have a little trouble getting used to it as sweeter milk is also a sign that it’s going off. (And if you share milk with someone else you’ll probably have to start using separate containers). I’ve added lactaid drops to my milk since I found out I was intolerant about 15 years ago. You do get used to the taste, just like you can get used to low-fat milk. You can also buy already-treated Lactaid milk. Look for this next to the regular resh milk; it comes in cardboard cartons. Unless you live in the UK, that is, where the intolerance isn’t very well-known or experienced. I have my mom ship the drops over from the States.
You’ll need to work out for yourself which products are the worst offenders for you. I and most of the people I know with lactose intolerance find straight milk and ice cream particularly bad. I can do cream, cheese, and yogurt without any problems, but other people I know can’t.