What Causes Delayed Periods Followed By Clear Vaginal Discharge?

2 days before my missed period i had clear slimy discharge and did NOT get my period.....2 days late i had dark brown spotting for not even the whole day and it stopped and then 2 days later it happened again.....i took 2 hpt and both were negative so i reinserted my nuva ring like normal and i havent had any period yet... i dont know what to think can someone help me please....i was on antibiotics 2 different ones but i used condoms and used my bitrh control normally