15 december 2020 i placed in the arm implant birth control but ive been having a light period for three weeks now.no cramping but my boobs are bigger and nipples are darker. is it normal?

Implant BC method ?: The implant BC method is close to the oral bcp for protection effect from pregnancy (over 95%), however , "light period for three weeks" is unusual for this progestin device. If prior ovulation occurred, and pregnancy is possible, obtain a serum HCG test to be sure about pregancy.

SIGNS OF POSS PREG: Although the hormone content of the implant may cause lighter periods, the breast and nipple changes are signs of pregnancy. I would take a pregnancy test ASAP and see the inserting doctor either way. Good Luck!