Hi there. I think jemma gave you some excellent things to look into that could be causing this. Do you get dizzy at all before this happens?
See a doctor, please, before you really hurt yourself! Blessings - Blu
I suggest you see your doctor to check you out.
You may be falling down due to muscular skeletal problems causing instability and a walking aid like a walking stick may help you.
Other reasons for falling down may be due to your blood pressure, either being too low or too high. And the doctor will take your blood pressure reading to see if this is normal or not.
Your iron levels may be too low. There may be a problem with your inner ear that regulates balance.
Make an appointment and get checked out.
Best wishes.