A due date of July 15 corresponds with an LMP of October 8, 2014 which matches your 26 weeks and 5 days (October 9).
You'd likely have been ovulating sometime between October 19 and 24.
Dates are based on your LMP and NOT when you actually got pregnant. You don't become pregnant immediately it takes several days.
It could be the "other guy." You'll need a DNA test to confirm.