Is she still having milk? How much?
Toddler portions are very small, smaller than most people expect them to be, but even so it does sound like she has eaten very little based on what you said she had today. Have you had her checked over by a GP? When you say she poops all the time, is it reasonably formed or small amounts of loose poop, have you had her checked for constipation?
WRT size and clothes, it's not usual for slim toddlers to need smaller sized clothes, at 2yo mine was in 0-3 month and 3-6 month trousers which stopped at the knee like long shorts. He is 3yo now and in 18 -24 month trousers with adjustable waist taken just about all the way in, plus sometimes a belt. He looks totally in proportion and has no weight issues, he also eats a lot so it is not lack of calories.
This is a guide to toddler portions but I stress it is only a guide and 'toddler' is a very wide range of ages/sizes (for instance mine would be classed as many more portions of fruit and veg and not enough protein and I've always tried to increase protein aiming for 1 portion per day rather than 2 as he used to have 1 portion maybe 2 or 3 times per week instead of daily)
milk 4 servings, serving size = half cup milk or yogurt or 1.5oz cheese, 1" cube
grains 6 servings, serving = half slice bread or quarter cup cereal/rice/pasta or 3 crackers
fruit 2 servings, serving = quarter of fresh fruit (or a handful, LO's handful not yours)
veg 2 servings, serving = 1-2 tablespoons
protein 2 servings, serving = 1 egg or 1-2 tablespoons meat/fish/poultry/beans