Can hair just thins out without any reason??

No: Many reasons including genetics, infection, hormone changes, thyroid problem are a few reasons. Ask your mother if she had this problem when she was your age.. It might pay to consult with a dermatologist.

Not usually: Diffuse thinning of the hair can occur with normal aging due to atrophy of hair follicles. Since pregnancy can stimulate hair growth, hair loss is common following childbirth. Traction on the hair from tight rubber bands can cause hair loss. A low thyroid level can contribute to both hair loss & heavy periods. If not done yet, your thyroid function should be checked. Best wishes!