The collapsing doesn't really sound diabetes related. It's a common trait of low BG but as a pre-diabetic this shouldn't be a concern.
If however, you're finding your blacking out, etc is following a carbohydrate based meal. It could be reactive hypoglycaemia. Unlikely, but possible.
You could request a glucose tolerance test if you wanted to. It's a case of drinking something like lucozade in the clinic and your BG is measured at set intervals afterwards to see how your body is dealing with the glucose. A fasting glucose test would help determine if your BG is dropping too low when not eating, however as you've been diagnosed pre-diabetic this would suggest your BG is typically higher than ideal.
As
@Bumblebee345 has said, anemia can make you feel extremely drained and unwell. You can request bloods to be taken at your clinic which will diagnose and/or rule out various illnesses.
Get well soon!
Grant