Could I be pregnant?? When to test?!

My last period was in July (1st day was July 20th). Sometimes my period is a couple days late and there was one time like 2 years ago that I skipped a month. But generally I'm fairly regular in terms of my menstrual. 
​Two weeks ago I went to the ER for back pain and they did a pregnancy test on me before they did the X-ray to make sure it wasn't going to harm me. It was negative. 
​My main question is, is there a chance I could be pregnant? Could that test have been too early for it to be accurate? Thoughts please!