<human>: Can you summarize the given text in a few sentences?

So i really need to tell this to someone, because i feel like i'm going to explode any second. Obviously a throwaway for this one.

It all started with her birthday a few weeks ago. Her ex decided to text her and wish her a happy birthday, which she was really happy about. She texted with him for about an hour or so, while being genuinely happy and excited (which seems odd, because he broke her heart, when he broke up with her). I didn't make anything out of it and just let it go. But ever since that day i've been having a feeling in my gut, that something is wrong.

A few days ago i saw her texting to a new friend she met (male ofc), and when i told her that i was a bit uncomfortable with her texting him (because he was obviously hitting on her, i know this because i saw some of the texts) she became really defensive about him saying things like: he is just a quiet guy, "he would never hit on me", "maybe he is gay". I convinced her to at least stop texting him, just for a while. Instead she went right ahead and deleted all of her messages to him and her ex.

And after all of her reassurances that she only loves me and that she would never cheat, i still have this really strong feeling in my gut no matter what i do. I did trust my gf 100%, but after her birthday something feels wrong and this feeling in my gut is just crushing me.

I don't know if this is relevant but her father cheated on her mother for a while, which my S.O. was really upset about.
To clarify, i am normally not a jealous type, but it just changed after the incident at her birthday.

I don't know if i'm just developing stronger feeling for her and therefore being more defensive and jealous or if she is really cheating. I need to know this somehow, because every day that passes by i just feel like shit.
<bot>: She text with her new male friend, i tell her that i am uncomfortable with it, she ignores it and keep on texting with him.