What Causes Blisters Over Knee After An Injury?

about a month ago I scraped my knee over the piece of plastic on the ground that holds sliding closet doors in place and later on, i picked the scab and it healed but left a scar. about a week later i shaved and about 2 days after i was scratching my knee and i guess i had scratched a blister that was formed. i didnt know it was a blister so the next day I peel off what I thought was dead skin and it kept getting bigger and bigger. its about the size of a quarter and a half and now its forming smaller type blisters about a centimeter away from it. I ve been applying dog wound ointment and pet anti-bacterial spray (thats all i have) on it and bandaging it for the past week and I m seeing little progress. what do I do now?