Chillin’ Out Pasta Salad

2 1/2 C (8 oz) medium shell pasta
1 C (8 oz) plain nonfat yogurt
2 Tbsp spicy brown mustard
2 Tbsp salt free herb seasoning
1 1/2 C celery, chopped
1 C green onion, sliced
1 lb small shrimp, cooked
3 C (about 3 large) tomatoes, coarsely chopped