My first thought was Purslane aka wild Portulaca, Pigweed, Pusley. The fat little paddle shaped leaves look right, and the flowers can be that big, but the wild ones are usually smaller. The unopened flower on the right looks like a purslane, too.
The hybrid purslanes you see at the garden centers have flowers in different colors incl. white that look like that, too. If you are near enough to find it again, taste one of the leaves. A juicy, slightly tart taste will seal the deal. (Purslane is a delicious and nutritous salad green.)
