you should get off the pill, all girls react differently on contraception, as im 18 i had the implanon, in my arm and had to get it out as my body didnt like it i had my period for 3-5 months at one stage got it out now just on the pill, so maybe try the implanon and condoms, be carefull and good luck
Addendum: But yes, you should see your doctor. If you're that tired, you may have anemia, and a simple blood test will detect this and the type, if you do. And regarding the getting your period no later than a few hours after being intimate with your boyfriend... Do you mean since this three-week thing started? That's how mine was at times, though I feel it was unrelated as I would still have a period on the days I was not with my boyfriend, just it would vary between spotting and being heavy depending on the day.
Have you been taking it so you have a period monthly or so that you have a period only every few months? I had a three-week period while taking it so I have one every three months, and my gyno told me to stop taking it, wait a week, then take it so I have a period every month for a few months. After that, I can go back to one every three. Apparently it's very common if you take it to have a period only every few months. Hope this helps!