Bowel movements that are softer than normal are usually considered loose stools. These can be a cause of concern if they happen immediately after meals or all through the day.
However, these cannot be considered as diarrhoea. The distinguishing point between diarrhoea and loose stools is that loose stools are soft bowel movements which can occur occasionally/ in regular intervals/ every day. However, diarrhoea is considered when you have loose/ watery stools frequently more than 3 times a day.