Thanks for your query. The gray tissue you are passing could be part of product of conception. I strongly suggest you do a repeat ultrasound
pelvis to rule out any retained products, meanwhile you can take antibiotics( recommendation is for DOXYCYCLINE), and for the pain take some antispasmodics like MEFENAMIC ACID/ any NSAIDS.
I hope this information has been both informative and helpful for you. In case of any doubt, I will be available for follow ups.
Wish you good health.