Thanks for the query.
The menstrual irregularity could be due to change in birth control as told by your doctor.
The black spot on the vagina can be a mole.
But all moles
are not cancerous.
If the mole is having features like: larger in size, irregular margins or elevated surface, altered color over the surface etc. that could be precancerous.
But don't worry that possibility is very less. Anyhow you once get examined.
For more details you can ask me through: http://www.healthcaremagic.com/doctors/dr-sree-gouri-sr/63429