Knowing that her current boyfriend is a guy you used to date, you are now someone on her radar.
In following you on Instagram, she is curious to see what your life is like.
This is where she gets to watch what you post and gain knowledge about how you think and live.
It isn’t unusual for a woman who is newly dating a guy to be curious about his exes.
The woman wants to get a better idea on what attracted her new boyfriend to said exes, believing that this gives her an upper hand.
In this case, you are his ex, or someone that he dated for a while.
His current girlfriend is following you on Instagram to build a mental portfolio on you.
She wants to have a closer look at your life to ascertain where there are similarities and differences between you and her.
She is looking for similarities and differences in personality, viewpoints, hobbies, physical appearance, etc.
She is comparing herself to you.
There is a curiosity to learn about where she is better than you and where you are better than her.
Being that she cares about him, she wants to do everything in her power to keep him.
To this end, following you on Instagram facilitates in giving her further insight into what you are about and why he was attracted to you.
Where necessary, she is intent on mimicking you.
This is especially true if she believes that this is an area that you are better than her.
For example, as she watches your stories on Instagram, she observes what you have a propensity to wear.
She is curious about it.
She suspects that a dress like this or that style of dress would be pleasing to her boyfriend.
Ergo, she goes to the department store at her soonest convenience and buys a similar dress.
The same thing applies for if she observes you wearing your makeup in a specific style.
Consequently, she changes how she wears some of her makeup to mimic you, thinking that this is something he would like.
All of this points to insecurity.
Basically, in following you on Instagram, she is demonstrating a level of insecurity in herself and her relationship with him.
She is worried that she isn’t good enough for him and that there are areas that she is deficient.
This makes her fearful of losing him as her boyfriend.
To avoid this, she goes about following his exes on Instagram so as to observe them and pick up on cues that she can mimic to keep herself attractive in his eyes.
This means that you aren’t necessarily the only ex of his that she is following on Instagram.
It’s possible that she is following a number of his exes on Instagram with the intent of mimicking them all to a degree.
This insecurity has another reason behind it.
In following you on Instagram, she keeps tabs on what you are doing online in reference to him.
This gives her an advantage in watching out for whether you are posting anything about him, whether it be directly or indirectly.
In keeping tabs on you, she gets to verify that there is nothing still going on between you and her current boyfriend.
She wants to confirm that you are not still hung up on him emotionally.
This confirmation gives her reassurance that you aren’t going to attempt to get him back.