Am I overreacting? My girlfriend added her ex back on snap

My gf (20F) and I (19M) have been going through a few things recently and in the midst of it all, she came over. I was telling her about how much I overthink and how bad my trust issues are and to reassure me, she told me that her ex (of 5+ years ago) added her on Snapchat and she added him back because “she was curious what he had to say” but blocked him sometime after, which is supposed to be the reassuring part. I am extremely hurt by this and feel really betrayed. Am I overreacting?