I'm experiencing hair loss. I've always had thick hair until over the summer when I got a bad haircut. What could be the cause?

Most likely it's not the haircut but timing that is causing you to hypothesize.Rapid hair loss needs to be evaluated by a dermatologist. The biotin may actually cause more problems if you are consuming mo're than 10mgs per day.Blood work, a good history,exam and possible biopsy would be helpful.Thank you

Bad haircuts, as you stated, don't usually cause any hair loss. Hair fibers are not living (they are just strands of keratin) - so cutting away hair has not impact on the hair growth machinery which is deep under the scalp. Bad haircuts don't cause hair loss. Does high biotin intake cause hair loss? The answer really depends on how high the intake was. Biotin is a highly water soluble vitamin. Usually excessive amounts are simply excreted from the body. However, extremely high levels could potentially cause problems. In order to determine the cause of your hair loss, you really should see a physician who specializes in hair loss. There are dozens and dozens of causes of hair loss.  For many of them, you can only determine the cause by performing an up close examination of the scalp and obtaining a very detailed account of the patient's medical history. Blood work and a biopsy is sometimes needed, but this is usually not necesaary. 

Hair loss in young men is usually (99%) genetic balding so you are best seeing a doctor who can build a Master Plan for the short and long term in managing your hair loss