What Causes Vaginal Spotting While HPT Is Negative?

hi im 20 years old i have 2 kids but my questios is i have irregular periods last year after i had my son i only got my period 3 times and know this year i got my period on feb. 12 but it wasn't my period its was spotting n light period and know i got my period again march 4 but its spotting again and i've token pregnancy test for a year in a half its say im not pregnant but i've try to get pregnant for a year and im not getting lucky and my husband is 41 is dat the problem