Not my ex, but he made it excuses for it. The girl he dated after me was insane. Never met her. We both moved on after a 4 year relationship. He started seeing her. I couldn't have cared less.
She started liking my Instagram posts and tweets after a few months. Then she'd undo it shortly after. Didn't think much of it. All my stuff was public. I chose not to look at her page because I didn't want to give in.
After a while I mentioned it to my friends. They looked at her Instagram and noticed from scrolling for a bit that around the time the weird liking/unliking pattern started, she was blatantly recreating my photos.
Now I know this sounds crazy, because it is, but she was straight up buying the exact same clothes and make up and doing her hair like me, same captions, weird shit. She even hunted Amazon for my favorite pair of gold heels I had a few photos in. They hadn't been sold in stores for 2 years (she bragged about hunting them down and ordering them) It didn't bother me at first, I actually thought it was funny. And creepy.
I ignored it for a long time until mutual friends of my ex and I noticed she was referring to me hatefully on twitter without using my name. I informed my ex after about 6 months of this. He said that she said I was a crazy person and making things up. Alrighty. From then on I started screenshotting everything. Any time I ran into the two of them at a bar, my friends got super uncomfortable because she would just stare us down.
Eventually I blocked her. She then started making fake accounts with different names and following me on them. I know this because she wasn't smart enough to use DIFFERENT photos than the ones on her actual Instagram. I didn't want to make myself private purely out of pride.
It got to the point where it was constant and I just wanted to move on from the whole situation. It was a painful break up and being reminded of it every day sucked. I got really mad at my ex, showed him the screenshots, and he agreed to tell her to leave me be. Once he did, the notifications stopped. Don't know if she stopped creeping for sure, but I would hope.
Shortly after that, they broke up and he contacted me to apologize. Turns out she was insanely insecure about how long our relationship was so she took it out on me. He found out everything I said was true and that I was not, in fact, the crazy one.
TL;DR - my exes girlfriend after me was a raging stalker psycho and he refused to acknowledge it for a whole year.