Workflow doesn’t evaluate properly the PRE function

Hello:

I have the following workflow (see screenshot attached to see all the details):

IfNull(PRE('{plPedido_OT_Test}'), "No") <> IfNull(FieldValue('{plPedido_OT_Test}'), "No") AND IfNull(FieldValue('{plPedido_OT_Test}'), "No") = 'Si'

I need that when a field changes its value and gets the value “Si” it sends a notification email. So far it does the notification, the problem is that after that, every change of every other field of the record fires the action again. It seems that the PRE function doesn’t evaluate properly that the field ‘plPedido_OT_Test’ has not changed anymore.