I had a 3 mm wide canker sore in my mouth that has healed about 3 weeks ago. however, in its spot, there is now a white line/spot/scar that is there since the 3 weeks. it's inside the skin. what can it be? is this a cancerous mouth patch or ulcer?

Cancerous lesions do not "heal" as you described... they would linger and look "angry". Your ulcer is most likely still healing, but for peace of mind you can always have your local dentist look at it.

Yes , it is Canker sore and is viral in .nature ,Must see your Dentist who can prescribe Antiviral Topical It is Herpes which can come and go depending on your stress and lack of sleep .It is contagious ,do not touch or rub on it .