The exercise in question is a type of reverse crunch. You lie on your back, put your hands under your head (or under your butt), put your legs out straight and lift them together until they and your torso form a 90 degree angle, then you slowly lower your legs back to the floor. repeat.
About 1/3rd of the time I do this, I notice that it causes pressure in my sinuses, centered on my nose and surrounding it. Sometimes the pressure radiates to my temples too.
My theory is that it isn’t the exercise causing the pressure, exactly, since it seems unlikely that lifting one’s legs could effect the head, but that I’m doing it by resisting the urge to pant from exertion - this is definitely the hardest exercise I do by far - maybe by clenching my jaw. Plausable explanation, or do is there something else going on?
For the record, I usually have an at least partly stuffed up nose due to allergies, so I won’t be surprised if other people who don’t have allergies don’t report this odd side-effect.