Hair loss can be due to stress but it's best to get a thorough check up as there are other things which cause hair loss in women.
The most common causes are anemia or long term low iron. I would get a full iron blood test and make certain they test your Ferretin levels. A lot of people who have low Ferretin find that one of the symptoms is abnormal hair shedding. It can also cause early onset of Alopecia to occur if you are predisposed to getting it in later life. If it is iron related it can be corrected, you just have to get your ferretin levels higher. Often doctors dont care so long as your test result comes back 'within range'. My test results were within range for approx 5yrs but i was losing hair and now that my ferretin levels are on the way up my hair has noticably decreased shedding for the first time in 7yrs.
When i was in the middle of my hair loss years i came to this board and learnt that it could be due to low iron levels. This is why i make the trip back to this particular section of the board to let as many people know about the connection between iron levels and hair loss.
It could also be thyroid related. Your thyroid can also be 'within range' yet you can suffer symptoms. If it's thyroid related it, your hair can grow back, the loss wont be permanent.
Sometimes it's due to insufficient amounts of a certain vitamin or mineral. When my doctor gave me a full blood test he included some things i had never been tested for and which i thought were unnecessary and then i was to find out i was extremely low in vitamin D and Iodine in particular.
It could be due to BC pills, but i would get a full check up just incase it's not.
Hope you find out soon as it's very emotionally distressing as a female to be losing ones hair. Best of luck and keep us posted