Blood in my urine but tested negative for everything

I'm 26 weeks & had excruciating lower back pain and cramps then I walked around and I felt a pulling pain on the right side of my stomach radiating to my right back. Husband took me to the hospital and they tested me for everything. They said I only tested for blood in my urine but the blood results were good. I have a history of kidney stones back before I even had my daughter. They said it's possible that could be it. Has anyone experienced this? Could they be right?