My period is 15 days late but tests are showing negative results

Girls, give me some advice, I’m seriously losing my mind! My period is 15 days late, but all the tests I’ve done have come back negative. Should I say it’s seasonal or something? But it has never been this late; it used to be like clockwork. I wonder if a cyst has formed or if it’s stress, I don’t know, but it feels like there’s some swelling in my stomach. Has anyone experienced something like this? Is there anything I can do before going to the doctor?