of course it is possible, but very unlikely if you take your pills as recommended (which it sounds like you do). I think it's about a 1%-4% chance. VERY unlikely.
Sometimes, your brain can actually make you believe you are experiencing pregnancy symptoms. Headaches, lower back pain, and exhaustion are all symptoms of stress.
Also, all of your symptoms could be because of problems with your pill.
If you had a negative pregnancy test, I'd be willing to bet that you are NOT pregnant.