Back pains & uterus pains

So my AF didn't show this month . I'm always regular . Never skip a month . Took four hpt all show negative . However I for some odd reason save all of my test . The four test I have taken shows negative until about a hour later there are faint lines on every single one . I've made my doctors appointment today . My uterus feels really crampy & my back hurts REALLY bad . Is anyone else going through or had going through this ? Or similar to my situation?