What Causes Reddish Color Urine With Stringy Item After Having Lithotripsy?

Hello,

Last month I had kidney stones, which I received shockwave lithotripsy treatment for, and I passed several small stones. Today, I had some reddish color to my urine, accompanied by a small stomach ache. In my last visit to the bathroom, I strained my urine to see if there were any left over stones. There wasn't a stone, but there was a small, almost stringy item in my urine. It isn't hard, its not thick at all. Do you have any idea what it might be? Should I be concerned?

Thank you!