Teeth pain but my dentist doesn't know what it could be

I've been experiencing high sensitivity in the back of my teeth, top row. I think it may have started when I moved to Montreal from Vancouver for a few months 2 years ago but I'm not quite sure.
The top back of my teeth has been really sensitive to mild hot/cold.
I was scared i may have root canal so i finally went to the dentist few weeks ago to get it checked.

After a few x-rays, they tell me my teeth is fine, asides from one which has cavity but it was on the bottom tooth which doesn't hurt. They say that some of my gum has worn off at the top and bottom but the bottom of my teeth does not hurt.


I got 4 fillings top, top and bottom back row (left side, going in for the right side soon) and I'm still experiencing sensitivity in the top row!!!!!!
I asked my dentist what's wrong with my teeth and she says she doesn't know!??!!?!?!?! Who and where am I supposed to get answers if my dentist doesn't even know?!
I feel like i just dished out $800 (and more if I get the fillings for the right side of my teeth) for something I didn't eve need.
I just want my teeth sensitivity to go away!!
Please help, any advice, possibly theories will help!

[Jerome Tsang, DDS]

Have you tried sensitivity toothpaste?  

How bad is the sensitivity and how long does it last when its exposed to something cold?  

[Baley3]

I am now with Sensodyne but its only been a week. Does it really work?

It lasts for a few seconds but it really hurts.

[Jerome Tsang, DDS]

It may take more than a week for the Sensodyne to take full effect.  

If you are experiencing sensitivity and are concerned about whether or not it may worsen, it may benefit you to go seek another dentist's opinion on the matter.  

[Baley3]

How long does it usually take to see results or to feel the full effect?

Thank you Jerome for the advices.

[Jerome Tsang, DDS]

I would give it up to a month.  If it worsens or lingers, I would go back to your original dentist or seek a second opinion as you don't want it to continue to linger and worsen potentially