In a medium bowl, toss shrimp with chili powder, cumin, and pepper. In a small bowl, stir together sour cream and 1 tablespoon lime juice. Heat olive oil in a large nonstick skillet over medium heat; sauté shrimp 3 minutes, turning once, or until done. Remove from heat; toss with 1 tablespoon lime juice. Fill warmed corn tortillas with shredded lettuce, shrimp mixture and diced red onion; drizzle tacos with lime crema.