This is tricky. Those all sound like promising pregnancy symptoms, but the negative tests certainly confuse things. If you can, I would grab a First Response brand and test with one of those. I had a friend who could get a positive pregnancy test until she was technically six weeks pregnant (and two weeks late for her period).
This is not medical, it's completely anecdotal, but I've noticed that the later my period gets, the weirder my symptoms will get, even though I'm not actually pregnant! If this continues to be an issue for you and you continue to experience nipple discharge, you should talk to your doctor because these can be signs of a very treatable thyroid condition. (However, if you've breastfed within the last three years, it's also possible that your body is just still randomly lactating - so fun - and something to let your doctor know about).