Upset and bummed about my ex, and her incessant quasi-stalking.?

We last dated a couple years ago in NJ. A regular social doyenne, who knows everyone who is anyone. But insecure.

I have since moved to MA. We have remained okay friends. We text a couple times a month -- basic stuff like "how's work?" But she is always ready to shoot some nastiness. Yesterday there were the most innocuous back-and-forth texts. Boring. Then she writes: "Truly D honestly it is fine if you are dating someone of course and who cares if I say it is fine anyway. No need to hide it. Someone sent me a picture. If you are dating her she's cute -- though young!!!"

It is true that I have been dating the girl for a couple months, and that she is young.

But this message really bothered me:

- 1st of all, it's none of the ex's business.

- 2nd of all, it must have been from something I put on Facebook last month -- just an uneventful beach pic -- but the ex and I are not friends on Facebook.

- 3rd of all, the ex is letting me know her friends (some of whom are mutual) are so loyal to her that they are taking screenshots of my account and sending them to her.

- Lastly, she is letting me know that she and her friends consider this age difference taboo.

I don't know why this is so upsetting. It freaks me out a little. Which, after all, is probably what she wants.

I think I'll just ignore her but I'd rather find just the right thing to say to stop this behavior once and for all, yet not come off as a jerk.

Any ideas? Thanks for your thoughts.