due date error

hi, I'm having struggles adding up my due date. please help! my LMP was 22/08/2017 and my doctor said I'm due 5th May according to this information. All of my apps I've entered the same date and they all say 6 weeks 4 days due may 28th or 29th. If I was due May 5th this would mean I conceived on the first day of my period. I have a scan on the 16th so will be able to update but I'm just so confused and worried it's all wrong. Thankyou for any comments! x