Lens Flare on Sand Dunes Exposure?

I have 3 ruined negatives that have what I think is lens flare. It's a subtle octagonal shadow on the negative. It was early morning and the sun was behind casting shadows on the dunes. DOES THIS SOUND LIKE IT'S LENS FLARE? I never had this occur before, and it's the only 3 negs. on the same roll ( the 3 best unfortunately). I was using a #8 filter...could it have been caused by the filter? Also, could it have been caused by the reflection off the sand? It wasn't extremely bright lighting as the sun was in and out of the clouds that morning. I didn't use a lens hood.....would this have averted the flare ( assuming that this is what it is)? I have several other exposures that don't exhibit this problem, however, they also don't have the contrast range of these 3 negs. either! Can't scan the neg. to post with this question-no scanner.
Thanks for any advice suggestions....

It's flare from shooting too close to a bright source of light (sun or reflection).

Are you using a lens-hood?

Filters will increase the likelihood of it happening, especially cheaper/older uncoated, single-coated or dirty filters.

Often, you can detect the probability of it happening before you press the shutter release: any light hitting the front of the lens/filter is likely to cause it. If your lenshood is unable to eliminate sun/glare on the front of the lens, try to stand in the shade of something (building, tree, steet sign, etc) and take your shot from there.

Used judiciously, they can add some interest to a shot.