Thank you both for your responses. My first thoughts where maybe a reaction from a bug bite (not an infection, but more like a welt or something), or maybe a kick from another horse (but it doesn't look like that kind of injury to me). Then the other people at my barn say they think I need to call the vet out. I blew them off, but later got worried about it.
LCharon- Did the horse show any other signs of infection?
trailqueen-this is our first year of ownership with this horse, so I have no idea what her history is. Never thought of allergies. If it is allergies and it doesn't get any worse or she doesn't have any other problems, shouldn't it be ok to just leave it alone?
Just judging from my feelings on it and your responses, it sounds like I did the right thing by not having the vet out. Wheew...I feel better. Don't get me wrong, I don't mind getting the vet out when it is necessary! It's just that unless it's an emergency, I'd rather have the vet out during their regular business hours (and skip the high-priced emergency call).