bottom of my teeth are black near the gums
This sounds like you have decay at the neck of your teeth. From your description given it is very difficult to know what it is exactly that is going on. I recommend you visit your dentist to have this looked at.
Dr Safeer Butt
S3 Dental – Synergistic Smile Studios
Clinics in Kent, Surrey & Sussex
Web: www.s3dental.co.uk Tel: 020 8398 6322