This song can be of more than one interpretation. Of course, this song gives the impression of a couple breaking up. Her skin is thick and she has an elastic heart, her partner can never break her. This love is ending, but she will still fight for herself and for her peace. She is becoming fed up with him, and his red flags (bad signs) makes her not trust him as well as anyone else.
To a point, this song gives another impression as inner strength for ones more deeper and personal reasons that doesn't directly need to mean breakups or relationships. It can be gaining inner strength from someone in whichever part of life who swallowed her whole, and left her feeling lost and hopeless; but by having thick skin and an elastic heart, she won't be down too long, and will come back up fighting for herself and inner peace.