Toadally not okay with how she's disrespecting YOU and y'all's relationship.
If this person truly cared about YOU and y'all's relationship, she would not intentionally do things that she knows are inappropriate and upsetting to you.
I'd draw a line in the sand and give her two options ... She immediately ceases any and all contact with this "ex", or she hits the road ... Period!
Oh, and about that "you're too sensitive" thing ... Did she tell you this? ... Often people who are doing bad things to others will tell them this in order to create self doubt about one's own perceptions ... Don't allow this person to mess with your head like that!
Sincerely,
Pfrog!