Wound Infection

My girl has a cut on her back right leg and it has some pus draining from it. It's not a large cut and doesn't look deep or anything. I don't know what she did to it or if she reopened the wound from awhile back when she jumped over the pasture fence. She is ornery about that back leg to begin with (it had to be THAT one!) so I went to buy an aerosol spray antiseptic for it instead of an ointment to rub on. I've seen cuts on her before but I've never seen it with pus. Should I be really worried?