Odd Sore/Spot please help

My family recently went on a trip (one week ago) for 2 days 1 night and when we returned we noticed our cat had a red spot on her left front shoulder. As of now the spot is more purple and slightly swollen. She is tender to walk on it on that leg, but will not show any sign of pain when you touch it or inspect it. We picked her up of the street when she was 10 and she is now 11. She was updated on all her shots, and was a happy kitty. We have tried to take perfect care of her. What could this be? Should we be concerned?

Photo:

Can't really see much in the picture. It sounds like she may have run into something and injured herself. I would take her to the vet to get checked out.

Thanks, I plan on getting her in by the end of the week.

She might have injured herself and it didn't heal properly and has abscessed. She needs to get to the vet ASAP for a check and possible antibiotic and drainage of the area.