My girlfriend commented on her ex's photo

The other day, while looking something up on my phone with my girlfriend, I accidentally saw the comments section on Instagram. She had commented on a post by her ex. What bothered me even more was how warm and sincere the comment was. I didn’t directly ask her about it, but it really sat heavy on my mind. What do you think should be done in such a situation? Is it normal for me to remain unresponsive?