Girlfriend was being secretive with her phone, tilting it away from me, and I took a look through it later on in the day?

She was texting 2 guys who I don't know, and they both had nicknames in her phone (not their real first names). Both were asking her to chill, and she deleted all of her replies. One of them asked when she got off work, then the next text was "beautiful." I asked her about it, and asked who the guys were, and she got really defensive and said, "they're just guys from work. Why are you being so insecure?" But she refused to tell me who they were... I am not an insecure person, but my gut instinct is very strong and it's been bothering me ever since. This was a week ago, and things have been good between me and her but it is still bothering me that she couldn't tell me who these people are... Advice?
Girlfriend was being secretive with her phone, tilting it away from me, and I took a look through it later on in the day?
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