In this thread -
http://www.diabetes.co.uk/forum/threads/help.123552/#post-1508890 - it wasn't a post meal hypo, it was a drop from 20 to 11, both levels that are well over target.
With mixed insulin you do need to know how many grams of carbs you are taking in, and keep that level consistent. Your mixed insulin dose will be able to keep your blood sugar levels in range for a certain number of grams of carbohydrate but eat more than that and you'll go high, or less and you'll go low. You should seek advice from your healthcare team on what carbohydrate intake your mixed insulin dose is designed to deal with.
A basal/bolus regime is an alternative insulin regiem that would allow more flexibility in carb intake along with blood sugar control. You haven't answered where you are based, but in England and Wales NICE guidelines recommend basal bolus insulin in preference to mixed insulin for type 1s.