It just means you're jealous and you still have some feelings for your ex. It's perfectly normal to feel this way. As long as you don't obsess over it, you proceed with your life as normal, and/or you find someone new, you'll get over it with time.
If you break up with someone and they get with someone else, what does it mean if I feel all weird - like angry at the fact someone else will be with them in the same way I was?
Lea is right...a little jealousy and hurt is bound to follow suite after your ex gets with another person. You feel like how they can move on so easily whilst you are still hurting.
Learn from them and do the same, move on and find someone else, sure it hurts but time will heal it all. But why waste time feeling upset when they are enjoying their life? Pull your shoulders back and relax, enjoy the things you have in your life right now and count your blessings. Show them how happy and confident you are... :) Good Luck and remember to smile!