Well usually the cause of a black discharge particularly if there is a nasty odour, is a forgotten tampon. Are you absolutely certain that you removed that last one when your period ended? The fact that you finished just 2 weeks ago tends to back up that theory.
It is possible that it might have gone quite high up in the vagina, so if you are uncertain please go to your practice nurse or gynaecologist and get them to take a look.
Other symptoms of vaginal discharge are as follows but I have explained the colour of the discharge for each condition.
Yeast infection usually has a thick white cottage-cheese discharge and red vulva. Gardnerella has a foul-smelling, thin discharge. Trichomonas has a frothy white discharge. Foreign body is obvious and has a terrible odour. Cervicitis has a purulent cervical discharge and the cervix is tender to touch. Chlamydia causes a friable cervix but often has no other findings. Gonorrhea causes a mucopurulent cervical discharge and the cervix may be tender to touch. Cervical ectropion looks like a non-tender, fiery-red, friable button of tissue surrounding the cervical area. Infected/Rejected IUD has a mucopurulent cervical discharge and the uterus is often tender.
I would advise you to see your practice nurse whether you find a lost tampon or not to be sure there is no lurking infection.
I wish you well and hope you soon get this sorted out.
With very best wishes
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