Am I overreacting?

Sam
Alright. My husband and I have been married for a short time now. Well this past week, his ex girlfriend from 7 years ago added him on facebook. She's now married with 2 kids. Well she's been pretty active on his page (I know it sounds petty). But she shared an old photo of them together while they were dating and commented about them. My husband liked it and started conversation. I don't know why but it really makes me uncomfortable. I don't bring up my past relationships and I don't talk to my ex's. My husband gets irritated with me when I tell him my discomfort. Am I out of line?