What Causes Pregnancy Symptoms While UPT Is Negative?

period last 3 hours then it's stopped. I'm starts gaining weight and having some symptoms like, morning sickness, always feel tired, and i also notice that's my breasts is getting bigger. I always wanted something that's sour, when i ate it i feels good but if i smell foods they make me want to throw up. I missed my PMS two weeks already but already took a home pregnancy test like two weeks ago and it was negative. My left and right back lower always hurts.