my cycle usually last for three days now it's lasting seven days just starting this week can u tell me why

Hello and thank you for sharing your question.

A normal period can last between 3 and 7 days.  Having a 7 day period may be irregular for you, but it is not unhealthy.  The change may be due to a change in hormones in your body.  No cause for alarm.  

If it goes longer then 7 days however, you should see a doctor.

Faith