What Causes Spotting, Nipples Darkening, Lightheadedness And Dizziness?

im 23 years old my periods are normally really heavy and painful and last 7 days til the last 2 i had which have ben very light on 1st day then spotting after i had pregnancy test about 4 weeks ago i would of been about 3 to 4 weeks pregnant but it come back negative i also noticed my nibbles are bit darker recently and get light headed and dizzy sometimes, im not on the pillor anything have been off the pill for about 4 to 5months i dont know whats wrong and im starting to worry bacause something dont feel right