Bald spot in-between cats shoulders

I got home from school today and noticed a bald spot on my year old cat. It was in between her shoulders and I was very confused as to why she had it. She is an indoor cat,and has not been outside since we got her. The only explanation I can think of is that she is allergic to the heartworm medicine we give her (Its called revolution) although we have given the medicine to her before and she has had no reaction to it. Her skin on the bald spot also has a red line down it,it looks like dried blood or a rash,I am not sure. We haven't taken her to the vet yet,and shes acting fine as of now. Does anyone know why this bald spot just appeared?

It coud be an allergic reaction to the Revolution.Sometimes an allergic reaction shows up after a few uses. One of my cats used to get bald spot on his head while on Revolution so the vet swtiched him to a different medicine. A trip to the vet isn't necessary but do let the vet know of the possible allergy and what other flea/tick/heartworm treatment you can use instead.