Motorcycle Safety & Driveability: Near crash on Honda CM450E, nylon backpack, honda cm450e

Question
I commute 40 miles to work every day on a 1982 Honda cycle. I always strap with bungie cords my backpack stuffed with casual clothes to wear at night when I return after midnight...cold in SE Ohio in October now!  

Today, I also had to haul a federal express box to work, and stapped it on the back first, then one arm strap of the nylon backpack.  13 miles into my 60 mph ride, the bike lurched forward and immediately slowed, like the brakes had just slammed on. There was no skid.

I pulled the clutch in an coasted quickly to a stop at the side of the road, turned to see my backpack wedged up under the rear fender and the federal express box with all the papers inside struck by a passing semi...all over the roadway.
At least I wasn't killed.
It took me about 10 minutes to pick up all the papers, and then to unwedge the backpack out from under the chrome fender. It had worn through and burned a hole in the lining of my coat from inside the pack.

The bike runs well, no observable thumps, bumps or knocks.
WHAT ALL SHOULD I BE CONCERNED ABOUT CHECKING ON MY BIKE: BEFORE DRIVING AGAIN...
BEFORE WINTERIZING IT TOMORROW.
Today was the last warm day in SE Ohio before temps drop too low to ride after midnight anymore.
Any Advice?

Answer
Kirk,
First of all, glad to hear that you and the bike are both OK.
The bike will live to run again no problem, these CB's seem to go on forever! I would suggest though, I know it seems a little patronising,get a luggage carrier for the back. They don't cost that much, around £20 (Whatever that is in dollars) and can be a real bonus, they provide more comfortable grab rails for the pillion too.
I would check your tyres for any distortion and if possible, check to see if the rear tyre has slipped on the rim. If that is ok and the brakes are still in good order, then you've got off very lightly indeed!
One other thing, check that any remains of rucksack, coat or anything else has been shifted from the mudguard and exhaust silencer, you don't want that horrible burning smell when you start riding again!
Good luck (Although it seems like you already have plenty, either that or a very alert guardian angel!)

Andrew.