Yes, this is normal. There is a connection between the movement of thought in the mind, prana in the body and the movement of the breath. When either one of these slows down, the others will naturally tend to slow as well. What you are experiencing is called Kevala Khumbaka in Sanskrit. Khumbaka means breath retention. Kevala Khumbaka happens when the suspension of breath is happening neither after a strong inhalation or a strong exhalation but simply coming to rest in the middle.
It is a sign that your body is preparing to enter samadhi.