What did I eat with sashimi?

Japanese egg omelet called Tamago

I believe you'd know an egg if you ate it... so I think it was something different... Most likely a pickled radish that's dyed yellow - Takuwan.

The pickled radish is slightly sweet with a slight crunch.

http://www.asianfoodgrocer.com/index.asp?PageAction=VIEWPROD&ProdID=1173

Was it sweet egg also known as tomago?

it's pickled radish. Japanese don't serve eggs with sashimi.

i would say that it's either tamago (japanese sweet egg)

http://prometheus.med.utah.edu/~bwjones/C741542647/E20070417140221/Media/Tamago.jpg

or danmuji, which is korean yellow pickles and is usually served with maki rolls, inari, or sashimi

http://www.maangchi.com/ingredients/dan-moo-ji

http://en.wikipedia.org/wiki/Danmuji