LOL truee....
There is a bigger chance of Mincheol giving it to her cause we know he did sports and even wanted to major in them and become an athlete before his injury than there is of her doing SH. At least there is no other foreshadowing to indicate that and I think it really doesn't go with her character so far, nor are the other characters showing it in any way with the way they are treating her.
Also I think that Mincheol gave it to her cause in the flashbacks from middle and highschool she is not drawn wearing it + Mincheol didnt have an injury then (as we found out from that friend of theirs from their hometown), so if he really did give it to her it was probably before college and when that injury happened.
I hope more things will be revealed at the reunion..
So apparently the FL do selfharm