So, when I was talking to Delphine. She mentioned the blades haven't found a dragonborn in over 200 years, about the time the oblivion crisis occurred. Now if haven't played oblivion, but could it be possible that your character in oblivion be dragonborn? Maybe he found out after the main plot. I don't really have any evidence though.

I doubt it, since we weren't able to wear the Amulet of Kings.

Could it be Martin, technically? Or no?

The Champion of Cyrodiil was no Dragonborn. He/she couldn't wear the Amulet of Kings or light the Dragonfires. He/she did, however, go on to become much more powerful than a dragonborn.

What Delphine was referring to was Uriel Septim's secret bastard son Martin, who was discovered in Kvatch but sacrificed himself in order to defeat Mehrunes Dagon before he could produce any heirs.

It would help if you played Oblivion. You would see that there is no way you are a dragonborn then.

This.