My niece is 3 & a couple of months old, she eats very little amounts of food & throw up sometimes. She's picky, I'm worried about her. No allergies. X?

Food intolerance? There are several types of food intolerances, and they aren't always due to allergies. Food allergies usually cause a rash as well as other symptoms, and it is uncommon to have several food allergies. These symptoms could be due to a number of different causes. See you doctor if her growth is affected.
Big differential. Food allergies are possible, so get tested if you want . Reflux and sensory issues might come into play, so have her checked by her pediatrician. Most kids this age with this problem are in power struggles, though. At three they have learned to potty and dress themselves and want to control what they eat. Offer only healthy choices (lots) and then let her eat what she wants. Don't fight over food.