my horse misscarriaged

Question
my horse was 4 months pregnant and has had all correct feed been riding everyday. she is in a paddock with a gelding that she bosses around all day. why might she have lost the foal?

Answer
100 reasons, lack of vitamins, being stalled, stress, not enough exercise, improper diet, genetics?

This questions is like me writing you and saying my wife had a miscarriage can you tell me why? Way too variables to even try to answer this. Ask your vet, they can do test.