Although rare, anyone can end up with a purple looking tongue i.
Veins take blood towards and away from areas in our bodies, so when areas are growing - as in puberty or pregnancy - they need more of a blood supply so all of a sudden you will notice these more than before! What dose of pred were you started on if you are still at 15mg after 7 months? Treatment may involve self-care measures or procedures by your doctor to close or remove veins.