the possible causes for your menstrual irregularity are,
-after removing contraceptive
device sometimes it will take time for regularization of periods,
-there is chance of pregnancy as sometimes pre-ejaculate
fluid can contain spermatozoa
means decreased flow during periods,
as sometimes urine pregnancy test kits are less sensitive during early pregnancy, you can go for blood test for pregnancy to rule out the possibility,
depending on the result you can proceed further with your doctor's suggestion.