Thanks for posting your query at DoctorSpring.com
If your last period was April 15, your due date would be 22nd of January, not the 29th.
It is virtually impossible to have conceived on the 20th of April, you would not ovulate the day your period ended.
I think it is more than 99 % sure, that you conceived from your fiance, as that is when you ovulated.
Due dates are not very reliable, and most babies are born a week or more off from the due date.
The remaining 1 % - well only a paternity test will tell you.
All the best
Please feel free to discuss further.