<human>: Can you condense the text into a few key points?

First time ever posting in reddit but don't know where else to turn. I feel like I may be over reacting but I'm still upset. 

Me and my SO have been together about 10 months. Before we started dating he was best friends with a girl we will call C. 

C and SO were as close as friend's get and hung out at least 4 times a week. Then my SO moved to a different city and met me; before we started dating at least 4 people warned me that C and SO were very close and I should be-careful. I looked of Facebook and there were a-lot of very close photo's of them together; ie her sitting on his lap and kiss marks on his cheek.

Naturally their friendship started to fade because of the distance and SO would tell me it was because he found her clingyness annoying. She has linked this friendship fade to me because it happened around the same time we started dating. 

She has complained to my SO that he is spending to much time with me and other people have told me that she bitches about me.

Every-time we come up to her city to see my family or his family she will ask to hang out at least 2 of the 3 days we are there and will get shitty with him if he doesn't.

She will call my SO and if he doesn't answer then she will call my phone and act like she wanted to talk to me but then after 5 mins of talking she will ask me to hand the phone over too him.

Whenever we all hang out together she will completely ignore me and talk constantly to my SO and never even look at me.

This weekend I came up to see my family and she went down to see SO. Over the night I got about 15 snapchat's of her and him hugging and stuff of that nature. The final snap had the caption "He's mine now". 

I really don't know how to react. 

I have been cheated on before by people who were "just friends" so I am worried that my past experiences are effecting my judgement
<bot>: My SO's clingy friend used to have a crush on him and ever since we started dating her clingyness is really worrying me