I (24M) is unsure if I am overreacting for my girlfriend (24F) making new friends

My girlfriend has been to bars/clubs with her friends and has been hit up by other guys. One of them got her socials and had texted her to ask her out to a dinner. I had actually saw the guy when I had went to pick her up so I think he is aware that she is already taken, yet he still asked her out.

She had assured me when she went to the washroom at the bar, the guy did ask her friends if my girlfriend is single and they told him no.

What pissed me off when he ask her out was that she didn’t reassure him that she already had a boyfriend but she just said she cant make it. After a few days he asks again and she agreed to have dinner with him.

Do I have a reason to be very upset about all of this? In her defence, he is a gentleman that night and she just wants to make new friends as given her current life situation she only goes to office and home. So her only friends are her colleagues and old mates. If I restrict her from going, it will seem like I am being controlling and forbidding her to make new friends and to explore more.

Side note: Her birthday is the day after their dinner so honestly for me, I feel like a sore loser who is waiting for girlfriend to come back from a date with another guy just so I can wish and celebrate her birthday