Can anyone tell me why my mare has not shown any signs of heat since she ad her foal?

My mare had her foal on April 18 and has not shown any signs of foal heat or her 30 day heat. We had to give her oxytocin to help her produce milk 12 hours after the foal was born. She is a good mom but can be a bit excitable. She has been beside the stallion every day, all day, since 8 days after her foal was born and has not shown any interest in him what so ever. Her foal was delivered without incident, and she is healthy and in good shape. This is the first time that I have had a mare that has not shown any signs of heat after having a foal and I'm at a loss to understand what is wrong.