Have you been smoking the same amount each time? If you smoke too much you'll have some intense highs when you first start smoking. It's probably a tolerance issue though I still hit a new level every once in a while.

But yeah your friend is right, THC breaks down into CBN and produces the "couchlock" effect and a more messed up feeling. This website claims that "CBN levels can be kept to a minimum by storing cannabis products in a dark, cool, airtight environment. Marijuana should be dry prior to storage, and may have to be dried again after being stored somewhere that is humid." It also depends on how early/late the plant is harvested overtime more THC will breakdown into CBN.