You seems to be really in trouble and require a thorough examination. It doesn't looks like vaginal infection
because usually infections does not produce bleeding.
You are having irregular as well as post coital bleeding. This can be due to any growth both benign and malignant, within your genitals. growth inside vagina and on cervix
are easily visible by naked eye examination, which i am sure your doctor must have excluded. Those inside uterus
, most common polyps can be detected only by ultrasound
In my view, your examination is incomplete without these. Go ahead and talk to your doctor frankly. Its your health.
All the best