Dog owners.. help me!!

Girls, I have 2 dogs.. 1 golden retriever aged 2 and 1 French bulldog 1 year in Feb. The Bulldogs a girl the retriever is a boy..

Just recently my girl French bulldog has been sitting ontop of my golden retriever and like humping his back?

Does anyone know what this is a sign of?

Shes already been on heat, and is definitely not on again..

Anything I can do to stop her from doing this?