Hi, I am quite empathetic the rash on the scar with itching after tearing the leg while sliding. Do not worry, if you had been immunized for tetanus then you will not have any problem. IF not then get immunized again at the earliest possible. You recollect if you have scratched it or anything new coming in your contact before appearance of the rash. I advise you to not pinch or pop the area otherwise these areas are very prone for getting secondary infection. For time being period you can apply antibiotic cream such as fucidin or mupirocin and take antihistamine tablet such as loratadine or cetrizine. Do not try to even touch or pinch the area as can cause more aggravation of the problem. You should preferably meet a dermatologist who can examine you physically and provide you the best treatment. Take care.