Is the hair loss patchy or generally diffuse? If patchy, the most common conditions are tinea capitis (fungal infection), Alopecia areata (autoimmune), or traction hair loss (do you wear a tight braid?). Try using a antifungal shampoo to see if it helps. Alopecia areata usually goes away on its own.
If diffuse, assuming you are female (based on your wanting body hair to be thin comment), it's almost certainly a hormonal issue. A classic symptom of hypothyroidism is thin hair, brittle hair, and diffuse hair loss. If you had been taking meds for hypothyroidism and those meds were lowered this could be the issue. It could also be a sex hormone issue. In either case it's worth a thorough discussion with your doctor and some blood tests.