Implanon works really well for some, and can both stop periods or result in breakthrough bleeding (which is what you have).
I have found that some women just get mood swings with any for of hormonal contraception - even though there is no evidence in literature.
i would talk to your Dr (or go to a sexual health clinic if you can - the staff are great and used to doing contraception!) about removing it if I were you.
Other options are a marina (IUD - uses same hormone, fewer side effects, lasts 5 years, cheap), Nuva ring (more expensive, needs to be replaced every month, you can do it) or the pill.
To remove the implanon you get a local anaesthetic around the site, a small nick in the arm and it is taped. Takes about 20 mins.
Hope your mood improves soon!