What Is The Most Common Cause Of Bleeding Between Periods?

Lily_I gave you a very long and complicated answer but worthy of consideration.  However at your age and if you are in good health, the most common form of "Breakthrough" bleeding is when you ovulate.  That is when you release an egg from your fallopian tubes and are most ready to receive the male sperm to become pregnant.  Women have different cycles of ovulation.  Some ovulate in the middle of their menstrual cycle.  Some ovulate more than once each month.  It is not uncommon to have a trace of blood to a tablespoon or so during ovulation.

Intermenstrual bleeding or bleeding/spotting during two periods is usually due to estrogen deficiency. If you get full periods between two periods or periods before 22 days gap then condition is called polymenorrhea. This can be due to endocrine disorder causing progesterone deficiency.  
One thing which I want to tell you is that no doctor or lab can go for hormonal tests in one day. You have to give blood samples for 2-3 times in a menstrual period. Because estrogen levels are tested immediately after periods. Prolactin levels are tested at mid of the cycle and progesterone levels are measured after ovulation.
In my onion, you should get hormonal supplemental therapy. For this, change your current doctor.