Pink fluid is known as spotting which is very common to occur while using certain birth controls. The person before gave a good comment but another note to add is the type of hormones present in your form of birth control. The implanon and depo provera release the hormones progesterone. THis is the hormone that makes up your endometerium, which is shed off during your period. So by consistently taking progesterone it will increase the buildup of your endometerium increasing the rate of spotting or having prolonged periods. To avoid this problem I suggest taking estrogen only or combination birth control (estrogen and progesterone) i.e. oral contraceptive pill
All women react diffrent to diffrent contraception its completely normal to get irregular periods or none at all i used the cerezette pill after having my son and that caused my periods to become irregular so i went onto the inplanon which i bled for a year strait so i went onto the depo jab in wich i stopped completely but had a two week period before the next jab was due so dont worry too much i suggest sticking with a contraception for at least 6 months for your body to get use to it and if problems persist than unfortunately you may find it best to try other contraceptives and find the one best for you if you are seriously worried i suggest speak to your gp as they can give alot more advice and checks if nessasary hope this helps