I have been in a relationship with her for 3 years almost. We both study and studied together.

Recently I found out she has been hanging out with this guy quite a lot. They spend 8+ hours together in the local library, go for food together, some days comes back at midnight, saying that she was taking a stroll with him and just chatting.

She went to his place a couple of times but she told me it was only for a short period of time, because he wanted to show her something. She is quite open about it, and she isn't hiding anything when I ask her about him. Is it something I should be worried ?