What Could The String Like Discharge During Periods Be?

my period is consisting in bright red blood and alot of

string like bits everytime i wip myself i have back pain feel incredably sick.it happens every few months and my periods only last 3 days most.my partner and i have been trying for a child for the last 3 years im not on any medication.i have been looking online to find out what it could be and all it keeps coming up with for results is miscarriage.and im really conserned