I admit I had an ironic wry grin on my face when I read your post. It is just hard to keep a straight face when your avatar shows a happy condom dude.
Anyway back to topic. I think it is extremely difficult for you to break the condom just from the act of cumming, that is why all condoms undergo a burst test. The condoms can expand to a balloon size.
Usually condoms break down from the excessive friction created in a hard and fast shagging. When this happens you will know as your cock's head will very exposed when you pull it out of her.
What I think happened was when you came in her pussy, you wanted to savor the feeling of her pussy
so you kept your now quickly softening cock in her pussy for too long. Then by time you withdrew your cock from her pussy, the condom had already lost its tight seal on your cock, hence allowing some cum to dribble out onto your leg. You are fortunate you didn't savor her pussy even longer as you would have left that condom behind in her pussy.
I always pinch the condom at the base when I pull out after cumming to make sure that condom is coming out with me. Girls go crazy when they have to fish a cum-soaked condom out of their pussy.
If you still don't feel any reassurance, then take the advice of the bros above and visit a clinic. Preferably don't visit your family doctor, there are a few clinics in the Wanchai area.