Need advice GP been useless!

Hi all I'm new to this so hopefully somebody on here can be of help to me! For the last 6 months I've been having reactions to sugar in food and drink.. within 10mins of eating/drinking I feel like I'm about to fall asleep and get bags under my eyes and feel really lethargic. I have a brother who has type 1 diabetes so this prompted me to go to my GP. I've been for several blood tests over the last 6 months and all have come back as perfectly normal, but I'm still having the same reaction to sugars. GP has been really unhelpful and has so far only suggested that I avoid sugar until late at night and use it as a sleeping aid!!
If anybody has experienced anything similar or can shed some light on the situation I would be extremely grateful for any input as I feel like I'm banging my head against a brick wall with my doctor!!

Hi daks thanks for your reply. it is all carbohydrates that I'm having a problem with, even eating a sandwich has the same effect. I possibly am looking in the wrong place but without getting any answers from my GP aside from being told to eat things that make me drowsy late at night because they can't find an answer, I'm having to do my own digging for some form of answers