Are you on any new medication? Thyroid medication will think your hair, so maybe doing an online search for thinning hair will show something that isn't immediately obvious to you.
Some shampoos will cause your hair to thin also, such as Pantene. Are you dieting? Do you take vitamins? If nothing comes to mind that may be contributing to this, maybe a visit to a dermatologist would be the best thing to do before it gets too bad and your options for treatment, if any are needed, may not do as much as it would now when it's just starting.
If it's enough for you to notice, then something is going on and you really need to find out what so you can stop it before it gets any worse.
Let us know what you find out, ok?