For the stock: Put the shrimp shells, chile, lemongrass, garlic, and shallots in a saucepan with 1 quart of water and bring to a boil. Simmer for about a half hour and then pour through a strainer. Only 1 1/2 cups of the stock is needed for this recipe. Reserve the rest by freezing.
Heat the oil in a sauce pan and add the curry paste. Stir fry for several minutes, stirring constantly.
Add the shrimp stock, ginger, and lemongrass to the pot and bring to a boil, then lower the heat to a rapid simmer.
Add the coconut milk and stir constantly to keep it from separating.
Lower the heat and simmer for 10-15 minutes.
Add the shrimp and cook until just done.
Add the tomatoes and cook for one minute so they're just heated through.