Pregnancy question

Has anyone started trying for a baby, and then couple weeks later had cramping and lower back hurting, followed by Breast hurt and getting bigger... but took a pregnancy test and it was negative ?

And Haven’t started my period yet

Granted we stared trying 4 days before my period last month and then I had my period, and right after my period we started again.. it’s been 20 days all together but minus my period so 15 days