What Causes Sharp Burning Sensation While Urinating During Second Trimester?

I am 18 weeks pregnant for the past week every time I pee it burns really bad and feels like something real sharp. My urine was tested and there is no sign of any infection. I had this problem about 2 weeks before I discovered I was pregnant and it went away in 24 hrs. I called my gyno and she seems to think that my cervical just be stretching n causing me pain but I don t believe that s the case.