Blood in my poo at age 20

Hi I have been having very light streaks/patches of blood on my stools for the last 3 weeks. I have been for blood tests and stool tests and all came back good. I am very concerned and have been stressing for weeks. I have Lots of gas and tenderness on my lower left side of my stomach. Don’t know if I should be stressing myself like this or not. 

Please can someone try and help me.