When you fire photons through the double slits, one photon at a time, they cause wave interference patterns with themselves as if each photon travelled through both slits.

You're right, I forgot about this part of it.

The interference pattern only appears after firing many many photons. They're still a photon (packet of energy) when creating an interference pattern. It's the distribution and probability of its location that changes. Not the physical "shape".

Edit: to clarify further

When not being detected, it's still just one dot that appears on the sheet. As more and more are fired, the interference pattern shows to show as each photon hits the screen

It's all about the distribution possibilities.