Your doctors are right when they ask you to to undergo tests to determine the cause or your bleeding.
There can be various causes for your vaginal bleeding,like
Thinning of mucosal lining of your vagina due to low hormone levels.
Any polyp or fibroid
Any cancer of vagina or cervix
Since treatment of each of these problems is different,it is very important to find the cause so that a proper treatment can be instituted.
There is no other way except to get tested.