I totally understand this, and recently went through it! An ex (who I have remained good friends with, just we know we are not good as a couple) just called me up and told me he is in love... I am so happy for him, but like you, my stomach was in knots for days!!! I thought there was something wrong with me too, since I knew I did not have "those" feelings for him anymore... I think it just brings up feelings of being replaced. Our emotions are not rational...there really is no explanation. It will pass, trust me :)
It's not true. Seeing him makes you think "That should be me!" and your jealousy is really of hate because you loved him before and your consciounce is stuck there and you're feeling that hate which has been washed away quickly with jealousy without you noticing. It's normal, I had the same feeling once.
Your subconscious said "that's mine. Tell it off
you were there first thats all that is, and at one point you were happy and seeing it shows that some one is taking away some of the happy you and he had made and she means more to him then you do now.
I think that you are probably just reacting to seeing him happy with someone else. Maybe you remember when you were happy with him at one time. Maybe you miss that feeling and not really him. You have a history with this person and it's completely normal to have those feelings. Sometimes too, we want what we know we can't have. Likely those feelings will pass as quickly as the wave began.
Honey, Jealousy is normal.
When I broke up with my ex, I spent ages checking her facebook to make sure she hadn't become any prettier. Stupid, I know!!
Time is a great reliever. You know you don't love him, so don't panic.
Probably more of a competitive thing than actual jealousy...it's a catty girl thing.
Human Nature, It is not real feelings, Just A familiar Reaction, cause you were with him.
Called being human. When you were with him, you had all those plans of a future together. Seeing that picture just reminds you those dreams are gone. Grieving for what could have been, not what you want right now.
Obviously you still have some kind of feelings or it could be you don't want him but you don't want anyone else to either.
Maybe cause he's in a relationship and you aren't? or if you are he seems happy and you aren't? Possibly? I know it sounds rude, text always does :( just trying to feel your honesty