Why did i have symptoms of pregnancy (a missed period, implantation spotting, nausea, enlarged and darkened nipples) but the blood pregnancy test said not pregnant??

What you consider a pregnancy symptom overlaps with other conditions. Some normally regular women skip a cycle or two at some point. By the time 60-90 days have passed with a negative pregnancy test many OBGYNs will offer meds to bring on menses. Pregnancy symptoms are triggered by the hormones that turn the test positive. If negative, could be to early, but is often something else.