There's more nitrate in bacon than in plain pork belly to be sure, but there's actually not much of it, and the amounts you need to consume to have a vasoconstrictive effect seems high. Meanwhile: bacon (along with all cured meat and fish) is one of the very saltiest things we eat. Lots of people are salt-sensitive. This is probably what virtually all claims of "Chinese restaurant syndrome" (MSG sensitivity) turned out to be.

You should have no trouble finding pork belly. Try an experiment: just make your own bacon by filling a pan with kosher salt and pushing each side of the pork belly into it until they're all evenly and aggressively covered. Wrap the meat in plastic wrap, dump the pan, and put the meat back in it; leave it in the fridge for 3-4† days. Rinse it off and slice it. See if you get a headache from eating it.

(Home-cured bacon is good but if you don't add some of the carcinogen back to the cure, it's not safe to keep much longer than fresh meat is).

† That's a light cure, but long enough for this experiment, and if you're new to baconing and don't know what to look for, under-cured is better than over.