“What causes my tooth sensitivity?”

Great question. So many reasons why teeth can be sensitive. Sounds like you may have switched toothpastes. Toothpastes vary in their abrasiveness. Whitening toothpastes are highly abrasive in general. That abrasiveness can lead to the sensitivity. If you have receded gums, that can cause root exposure and those roots are very susceptible to being sensitive. Your bite or occlusion can be off which can lead to microfractures in your teeth and sensitivity. If you have undiagnosed decay, yet another cause for sensitivity. Medications can cause your teeth to dry and lead to sensitivity. A root cause is that you have a breathing disorder which can cause some or all of the above and lead to sensitivity (few know of this one). I could go on, but my recommendation would be to visit a local dental professional for their expert opinion. 
Best of luck.

My first recommendation is to see a dentist. There are many reasons for tooth sensitivity, only one of which is helped by anti-sensitivity toothpastes.