Thanks for query,
As your pregnancy
test shows negative result there is no chance of pregnancy.
when we are on birth control pills & when we stop taking it we have hormonal
imbalance & because of it you get irregular pattern of bleeding.
just wait for few more cycles it will become regular.
if it will not after 6-7 cycles then consult your doctor to rule out any ovarian lesion.
If you are sexually active take other measures for birth control.