All humans lose their ability to digest dairy products after they are a few years old. Being lactose-intolerant at an adult age is normal.
Only Western-European people mutated (a few hunderd thousand years ago). They keep producing lactase for the full duration of their lives. Asians, Africans, etc, will become lactose-intolerent at the age of 5 or so.
So it depends on your race.
Dairy products are in everything. Lactose is a very cheap filler. And it is used as filler to produce many medicine-pills. It's used as filler in candy. It's used in dough (cookies, pizza, anything you can think of). If you are really lactose-intolerant, you should have problems with almost anything you eat. Well, over half of what you eat. In my country (The Netherlands), milk is seen as the most healthy thing ever. And we put dairy into almost everything. Also because it's so cheap. USA might do it a little less. But still, over half the products you eat might have lactose.
There are tests. If you are lactose-intolerant, your small intestines will not produce lactase (enzym that digests lactose), and thus not digest lactose. Lactose will go to your big intestines. Bacteria will digest the lactose. That will produce a lot of gas (Hydrogen).
Which can be measured in your breath. Go to a doctor. Do the test. It's quick, it's cheap. Make sure you know exactly what your problem is. Because there is so much lactose in food, it will be very hard to emperically find out yourself.
I've had a
parasite in my small intestines. My doctor totally underestimated that, and said my belly-aches were psychological. Some tests were done, but the parasite was not found. It fucked with me for 6 years, maybe longer. In the end, when I switched doctors and we finally found the parasite, it turned out I was totally lactose-intolerant. (And not taking in vitamine B12 and other nutricients. My gallbladder was fucked. (Making me always feel nauseous, probably because of the parasite too). A 5-day anti-biotics removed the parasite. But my intestines were still fucked. It took 4 years until my lactose-intolerance disappeared (the wall of my small intestines had repaired itself) and my B12-levels were back to normal.
So my advice:
Check to make sure lactose-intolerance is indeed the cause.
Get checked for parasites and bacteria.
If that is the reason for your lactose-intolerance, it can be reversed.
If you are not of Western-European origin, there might be nothing you can do.
Hope this helps.