Aatrox squishy bug?

Is it just me or every time i play , i feels very squishy even if i always build tanky items first?

He has a higher armour/mr/health than most champions at level 1, but the feeling i get with him is everything deals way too much damage to him?

I just had a game in aram and i built full tank, almost have double mr/armour compared to my teammates, yet it feels as if i have no armour. So when i clicked on the death report, i found something interesting

okay so i adds all the damage the abilities/basic attacks did to me, a total of 700 some thing( did the calculation right trust me ) , then on the bottom of death report it says 1788 damage in 1.8 seconds. Then i founds out i have 2800 hp. So how is this even possible? Unless i used up 1088(1788 - 700) health on abilities?(wow) But i had 2800 hp? Where did the rest 1012 (2800-1788) hp that weren't damage dealt to me by basic attacks/abilities, not used up by my self casting abilities( assuming i used 1088 hp on casting abilities ), go? And when i compare the damage taken from me and teammates, e.g. heimerdinger building straight ap taking a alpha strike, with me over 220 armour taking damage from alpha strike, the damage was about the same? Is it a bug?

But then on second thought, i'v seen other people playing Aatrox who build tank and they don't seem to be squishy at all, could it be that i'm just bad at him? Then on third thought, i'v been wu against Aatrox before with a e,q combo chunking away 1/3 of his hp at level 2? Any one else getting the same feeling that something fishy is going with how Aatrox's armour/mr/health works?

Sorry for soo many question marks i'm really confused :/ Gonna experiment more on it tomorrow.