tender breasts

as of recently both of my boobs have been very tender and sore to touch, i haven’t been wearing any bras since the start of the year, but i do wear those stretchy elastic t-shirt/gym bras whenever i go out.

i’m currently on the nexplanon/implant for the last two years and haven’t had a period since the insertion (617 days late).

just wondering what this pain could be? a start of a period? hormonal changes?

(ive taken a pregnancy test and it came out negative, the last time i had sex was 4 months ago)