That's what carbohydrates do to your sugar levels. Avoid them the best advice. If it happens though, drink plenty of water and exercise or go for a long, fast paced walk
Short answer is that you are eating a lot of carbohydrate, the shredded wheat, the granary bread and quite often the oatcakes, will all affect your blood sugar levels.
I'm not sure if you've had the information about carbohydrate that is usually given to newbies, but will tag @daisy just in case, she has some great info to help.