There could be hormonal imbalance. I would speak to your doctor about possible causes and get blood work done, particulary your thyroid. If the thyroid is off there can be hair loss. If you have some sort of imbalance you can then address it. I also had an experience where prescription strength tagamet turned out to be the cause of my hair falling out in a big way. It could be a side effect of a medication. Oddly enough the drug's posted side effects did not include hair loss, but my doctor said it could cause hormonal changes and cause hair loss. When I stopped the drug my hair stopped falling out and grew back.
That being said, I also use a shampoo called Nioxin. It's excellent and made to help with hair thinning. It can be purchased in some salons and those beauty supply stores. You can probably find it on line if you google it. It's pretty expensive but it's great stuff. By the way, I am a female.