It looks to me like it could be milkweed leafminer, Liriomyza asclepiadis:
http://bugguide.net/node/view/...
If they haven't already left, you may see tiny maggots underneath the leaf's "skin", if you don't want to try surgery to find them then shine a flashlight through the spots and see if you can see them that way.
It may help to know where the plant is located (e.g. country, state/province).
