In a medium saucepot, heat fish stock with shrimp shells and simmer for 15 minutes. Strain soup and discard shells. Return stock to saucepot.
Mix chili sauce, fish sauce, lime leaves, lemongrass, Thai chilies, and sugar with the stock. Bring to a boil and simmer for 2 minutes.
Just before serving, bring soup back to a boil and add the mushrooms and shrimp. Cover and turn off heat. Allow the shrimp to cook through, about 2-3 minutes. Uncover and add the lime juice to taste.
Recipe by Jackie Ourman at http://jackieourman.com/thai-hot-and-sour-soup-with-shrimp-tom-yum/