Im 20 yrs old guy and 2 weeks ago I saw the hair in my left temple looked fine. It’s thinning out really fast and I dont know what’s wrong. My hair in the left temple is thinning fast.I noticed this after getting head and shoulders. I used a comb to scrub my head with H&S (I tried to clean something out of my head) Could this be the cause? My temple thinned in a 2 week span. Also I notice I have about 2 scars in my left temple now.

Shampoo isn’t going to cause scars on your head. It’s possible that you had the scars, but they were covered by surrounding hairs and not noticeable.
If by “temples” you mean the corners of the hairline, the loss you’re describing sounds like a maturing hairline (particularly at 20 years old). Many people call the hairline corners the “temples”, even though the temple peaks are actually the points between your hairline and your ears.


Could a head band cause hair loss? I know the answer will probably be “if it pulls on the hair it may cause traction loss” which I understand, but I honestly don’t know if something like this has potential to pull on the hair. 
I am sorry that you are having a hard time with hair loss. Losing 20 hairs a day is very normal, as one can lose 100-200 hairs a day and this can be normal in many men. In its place new hairs are growing without you even noticing it to replenish the hairs that have fallen out.