What Causes Pregnancy Symptoms While UPT Is Negative?

i'm 23 years old and my husband and i are trying to concieve. my period is always regular as clockwork every 28 days on the dot for 7 days and has been for several years now. my period was supposed to start on sun june 26 but it was over a week late starting on mon july 4. about a week prior to and during the week when my period should have started i was having mild to sever back pain and lower stomach cramps, break outs, dizziness, insomnia, fatigue, nausea, mild mood swings, a vastly increased sex drive, and breast tenderness. when it finally came on the 4th it was lighter and shorter than normal lasting only 3 days with no period symptoms. the stomache cramps and back pain are now gone but yesterday i began spotting dark red almost brown with slimy discharge. ive only noticed it twice. once yesterday morning and again tonight. i took a HPT on the 3 day of my missed period and the results were negative. should i take another HPT just incase or wait it out?