Mix lime juice, sugar, and fish sauce in a small bowl.
Heat oil over high heat for about 1 minute, add the garlic and cook just until the garlic begins to brown. Then add the lime juice and red pepper flakes, mix and cook to reduce the mixture - about 3-5 minutes. You should end up with a syrupy mixture that is only about 1/4 the original volume.
Add the shrimp and cook, still over high heat. They take about 2 minutes per side.
Garnish with cilantro.