Shikantaza in original usage means ~'only sit', and the term was introduced into fairly standard usage by Dogen Daishi c.1250AD. This is in the context of Original Traditional Authentic Soto Zen, where the term is fairly similar to zazen, which means ~'sit & contemplate' etc, and so, within context, the two terms authentically mean about the same thing: sasaru means to sit/squat, & is pronounced 'za' in that sort of word construction, and zen means ~think/contemplate. In the term shikantaza, the 'zen' part is inferred from context, and shiken in this context means ~only.
Traditionally, in Soto Zen, shikantaza & zazen aren't directly about concentrating on breathing or on the nose or on relaxing, etc, and shikantaza more explicitly indicates that with the 'shi' meaning ~'only' part. So maybe the Asker was participating in something that was was similar but might have differences in meaning and of what's meant by the terms zen & shikantaza!
This answer is from the context of The Traditional Soto Zen instructed c.1250AD by Dogen Daishi. There may be other places that use the term shikantaza and the term zazen differently. Nice interesting question.