Any idea on what these red circles might be?

[king_nick1212]

My girlfriend recently (2-3 months ago) began developing a red circle (almost perfectly round) on her belly.  It's always in the same spot and gets lighter and darker from day to day.  Some days it's barely visible and then a few days later it reappears. Any idea what this could be ?  Seems unusual to be a food allergy ei: Gutan/Lactose  since she hasn't really changed her diet in 27 years.  She did recently begin a new birth control pill.  Any advice helps.

Thanks!