when you eat you need to eat slow release high energy foods to keep you going. otherwise you will get tired. feels like your trying to move through jelly. also if you dont already i would suggest a multi vitamin.
you might be lacking in iron too, that can make you lethargic, eat lots of dark green leafy vegges and have some vitamin c with them because vitamin c helps you to absorb the iron.
stay safe
xo
chubbymummy