Should I get counselling for my jealousy?

I know this is very silly and irrational, but I sometimes (not often) get jealous of my partner's past. He was married before he met me and still sees his ex-occasionally as they share a child.

I also flew off the handle the other day as one of his old friends who he had casual sex with years ago liked a photo of our daughter on Facebook.

Pathetic I know. But I've never had friends with benefits so I just don't understand how you can carry on being normal friends after having a sexual relationship. Why do I get jealous of things that happened before me? It's so silly! Should I get counselling?