I have what seems to be lineanigra from bellybutton to pubis, neg hpt, lmp10/28/12 no sex since 9/2/12. can lineanigra show w/o bein preg?im dark skin

Yes it can.: Some dark-complected women have a visible linea nigra whether they are pregnant or not; it may have been there all along and you only noticed it recently. While it is unlikely that you are pregnant it's not totally out of the question. If you're sure that the line wasn't there before and now it is, you should see your gyn to rule out pregnancy & hormonal issues, e.g. Adrenal insufficiency.