What Causes Implantation Bleeding,spotting,weight Gain And Enlarged Breasts?

I think I am pregnant but all my tests are negative, I have had all the signs including implantation bleeding, spotting, weight gain, bigger boobs and more.. I am in college and my parents told me to drop it and do my homework. I went to the health center and all they could tell me was pregnancy tests are 100 percent accurate when I read they are not.