Hi Fady,
I know it's been a few days, but we were hoping you'd respond to @oldturkey03 and post some pictures.
I ran into basically the exact same situation on my daughter-in-law's iPad mini, so I speak from experience here.
If you're flexing the case, and instead of it just straightening out, the screen starts lifting, then the most likely diagnosis is that your battery has swelled and is exerting pressure on the case and the screen. It's unusual for the case to bend; usually you see the screen coming up first, but it's certainly possible.
A swollen battery is fairly serious and should be replaced immediately, as it's a fire hazard. I mean that literally; the battery can actually explode - ask the people who owned a Galaxy Note 7.
So in addition to straightening the case (if needed), you'll need to open the iPad up to check the battery. An intact battery will be flat and rectangular, while a swollen one will appear pillowy.
In order to straighten the case, follow the battery replacement guide up to Step 44 and stop there. At this point the delicate LCD and glass digitizer have been removed and you can safely flex the case to the point where it's straight again. Of course, if you find that the battery is swollen, then you'll want to continue the guide to remove it and replace it with a new one. If you do discover the battery is swollen, my bet is you won't have to do anything about straightening the case, as it will most likely have been the battery deforming it, and once the swollen battery has been replaced, it will probably fit back together just fine.
[guide|88138|iPad 5 Wi-Fi Battery Replacement - iFixit Repair Guide|new_window=true]
You might want to preorder some screen adhesive if you're going to open it up since you'll need it to put the screen back on whether you replace the battery or not.
[product|IF128-005|iPad Air, iPad 5, iPad 6 Adhesive Strips - iFixit Store|new_window=true]
Of course, should you need one, you can also order the battery from iFixit as well and in the process support their repair efforts.
[product|IF128-027-4|iPad Air A1474/A1475, iPad 5 A1822 and iPad 6 A1893 Battery|new_window=true]
You can order the battery by itself, but if you haven't done a lot of repairs, I'd recommend the Fix Kit version as it comes with all the tools you need plus the screen adhesive as well.
Good luck with your repair; let us know what you find!