48 days since last period, mild lower back pains started today. negative test on 1/22. what could be wrong with me?

Hard to guess: In a woman with regular periods, some causes of a late period include: excessive weight loss or weight gain, increased exercising, emotional stress, having an illness, traveling, medicines (like birth control pills), hormonal imbalances, etc… Seeing a doctor is a good idea if symptoms are persistent, recurring, or worrisome. Home pregnancy tests are very accurate by three (3) weeks after sex.