I just started having sex in October and I supposedly had a period 11/15-1/19. It was abnormally light. Usually my periods are pretty heavy to light and have constant flow. Last time it was heavy on day 1 and the rest was light and sometimes not a constant flow. A friend told me that when you start having sex, it can make your periods irregular. Is this true? Or could I be pregnant? I tested for it on 11/21 and it came negative. Would it be too early to test if the last time I had sex was on 10/31? I haven't been having any symptoms.