OCD -- Obsessive compulsive disorder.....
Kitty, it must be extremely difficult putting up with a nuisance ex. I'm betting that she's very curious about you because as a mom, she wants to know who she's exposing her kids to (oh, and it doesn't hurt to make the ex suffer and cringe a little in the process...lol). Try the simple approach of talking to her directly, answer her questions and then go from there. Stay calm and let her vent her frustrations. Give it an honest-to-goodness good faith go, and then if things don't work, you can take other measures to keep her away from you. The relationship she has with your bf is one thing that isn't going to go away because of those kids. She needs to view you as a separate individual. Give her that chance. I bet you'll be able to calm her concerns.
When all is said and done, the one(s) most impacted are the kids. I hate to say it, but this won't end well if the kids become terribly traumatized by their parents fighting over gf's and bf's. Maybe its up to you to take the initiative and approach this woman yourself in a sincere and civil manner. Invite her for coffee and crumpets... lol. You might be surprised at what a woman-to-woman talk can accomplish!
I hope everything works out for you!!
Dr. Philuffy