What Causes Concentrated Urine With Solid Particles In It?

I have a rash on my back that resembles acne in fact that i what i thought it was. I have a pressure on my left side about where my ribcage starts. I have been urinating a decent amount but it seems concentrated and seems to have something in it like white sand. I thought it was just bubbles but under further inspection it seems there's something solid in my urine. It was thought i have kidney stones, but when i went to the emergency room i urinated and the doctor thinks we lost the stone. So, its unclear if i have stones or not. He put me on antibiotics for 3 days. (2 pills for 3 days) and he sent me on my way. What should i do? should i get an x-ray or get checked for a UTI? My doctor told me i have a "high concentration of bacteria in my urine" if that helps what-so-ever?