I really do think it is something from the very high humidity and warm temperatures you've been having Jelinda.
It kind of looks like what happens to roses when they get black spot, which is a fungus. It is in the tissue of the leaves. Not on the surface. Plants are susceptible to this type of thing when they are stressed. The combination of high humidity and heat is what will cause the fungus to start growing in the tissues of the leaves.
As your weather gets cold this should disappear. The affected leaves will die, leaving the healthy ones to carry on.
