adding an article to the cart

Sometimes i get the following:

i add an article to the cart, then a popup comes with say article added to cart, do you want to shop further or do you want to checkout.
the first time this goes allright.
But when the checkout is finished and you go back to the shop because you want to buy something else suddenly, you add an article to the cart, the same pop comes, but when you select goto the checkout it suddenly say the cart is empty, is there some kind of timing?

With regards

Guido