My whatsapp often shows me as having been online as seen by the "last seen" status, when I have not been on line. Could it possibly be related to my phone Samsung A3, showing me my whatsapp messages as a preview when i receive messages, with the option to close or open message?
No. It is not related to phone problem.
When your WhatsApp in foreground then it will shows as online to others. When it is in background (After removing from recent tabs), it will shows last seen to others.
Sometimes if your WhatsApp in recent tabs then it will shows online to others.