Hairfall of upto a 100-150 hairs in a day is a normal thing and you shouldn't stress out about it. as stress will lead to further hair loss and again stress. this cycle will continue.
causes of hairfall could be - certain dietary deficiencies like biotin deficiency.
scalp infections like certain bacterial infections or fungal infections can cause scarring and therefore hairloss.
weather changes, genetic factors, hormonal imbalances etc.
get yourself checked out by a dermatologist and establish a exact cause of the hairfall. then some treatment can be started.
till then just cut down on your stress, eat healthy and biotin rich foods. practice meditation and yoga and you will be fine.