An Otaku is a japanese word for a nerd. The word 'Otaku' is only used outside of japan as a self-label for weeaboos who think using a japanese word makes them cool. It doesn't.
If you do not refer to yourself as an 'Otaku', then the person who called you one is a weeaboo and needs to use english words ie. Nerd or Geek.
For some reason wapanese think being an 'Otaku' is a good think to be proud it when actually in Japan "Otakus" (english translation: Nerds) are frowned upon in the society and are seen as a burden.

TL;DR: Only you yourself can label yourself as an Otaku. Others would just call you a nerd, unless they were weeaboos who are complimenting your love on shinyshinykawaiiNippondesudesukawaii