In any emergency or urgent situations such as allergic reactions or sudden illness, always contact your emergency services as quickly as possible.
For less urgent issues, your first point of call should be your health visitor, doctor or paediatrician.
For problems with feeding such as fussiness or a child eating only a limited selection of foods, you should contact a specialist in infant nutrition, usually through your health visitor or doctor.
If you wish to contact a nutritionist privately, you may find details of local practitioners online, for example at your national society of infant nutritionists.
Most countries have useful websites from national government health departments providing up-to-date guidance on infant and maternal feeding and nutrition.