so confused.. (Long post)

I was on Depo for 3 months (1 shot), I didn’t go back for my second shot because I legit bled the entire time & lost so much hair! This was back in April when I stopped it. So I’ve been on no birth control for 3 months & I’ve had normal periods UNTIL July… I was due for my period July 22 & it never came.. I’m currently 18 late & tests are negative.. I haven’t been anymore stressed than usual, I’m not taking any medication, my weight & diet hasn’t changed. I have 3 littles already & this is just really confusing.. has anyone had this happen before? Could the tests be false negatives?