Could I be pregnant?? When should I test??

For the past 5 days I have been having back pain only during the middle of the night while I sleep! It all started after I had a sharp pain in my uterus that woke me up out of a dead sleep. I'm hoping that could of been implantation! Today is day 28 and my doctor says I can test today but my cycle doesn't normally come til day 32. I'm so nervous to test because my husband and I have gotten nothing but negative test for over a year!! When should I test???