Breath getting shorter and shorter

Practice : Meditation using breath I'm practicing sitting meditation using breath according to Thai Forest Tradition method. Mentally reciting of "Buddho" . I practice at home so would like to get help based on my experience so far I noticed my breath getting shorter and sometimes period of no breath or "nano/very short" size of breath . **When it is a series of "nano/very short" size of breath , mostly what I experience is "waiting" for next "in" breath ? Is this right or where should I concentrate during such occasions ?**