I (21) and my GF (21) have been in a six month relationship.

My girlfriend recently started texting a guy that is in our martial arts club. At first I didn't mind but she begun texting him every day. The texts at first were just memes but then she started adding hearts and cheesy messages such as "I will miss you :( ", "How are thooiii?". She was doing it even while we were spending time together. I don't have any problems with her having guy friends at all. Yet, she has known him for only 4 months and they only recently started texting. She even started asking me questions about him, i.e. "Was he at training today?", out of the blue. I told her that I don't like that but she claims he is just a friend and that she texts everyone like that and its normal. I find it very disrespectful and yes, it makes me feel insecure. What should I do?

TL;DR! - "GF started texting another guy everyday and I don't like it"

UPDATE: Thank you all for the replies! I confronted her and she stood her ground saying that it is how she texts her friends and that "friends share feelings together and support each other". She told me that I am being insecure and that she doesn't like him and even if he asks her out she will reject him. I told her that I am breaking up with her. And after that she has been contacting me to discuss again. She says that she won't do it again and wants to be together.