What Causes Missed Period, Frequent Urination, Lower Back Pain And Lower Abdominal Cramps?

Hello, so I ve missed my period, I took a HPT and it came back negative, I took the test a day after my missed period. It s now been a little over 3 weeks and still nothing. I haven t yet tested again. But the symptoms along with my missed period are frequent urination, lower back pain (located at almost the very end of my tail bone) slight cramping in the lower abdominal area, I also get heartburn a few times in the middle of the night which isn t very uncommon for me. Just curious as to what I could have this by if not due from pregnancy, or maybe I just tested to early in the beginning? Thanks in advance.