Normally when you get a Bitter tasting smoked meat it is creosote on the meat. If you get a numb/tingling feeling in your mouth or tongue its definitely creosote.
There are several things that can cause this;
1. stale smoke IE smoke not moving threw the smoker.
2. smoke too thick/ too much, you want thin blue smoke out the vent, not thick white smoke keep in mind if you can smell it your smoking.
Here's a tip get yourself a smoking tube or tray from A-MAZE-N Products, LLC they produce thin blue smoke. I use a 12" tube in my Master Forge 2 door propane smoker, and have NEVER had a problem since I started using 1.
3. Green wood, smoking wood needs to be seasoned/dried before using it to smoke. Once it is seasoned you can soak it and use it.
There are other things that can cause this but these are the most common.