[quote=@Game Maker]
<Snipped quote by Jinxlynx>

A fighter's HP after 1st level is 1d10(or 6 if you roll under that) plus the constitution modifier per fighter level beyond level 1. My constitution modifier is +2 and I rolled a 9, so at level 3 my HP  would be 9+2+2=13
[/quote]

It does not work that way though. At first level you take your full hit dice so even with a CON of 10 which has a modifier of 0 your HP at level 1 is 10. Then for each level beyond level 1 you roll your 1d10 and then add your CON modifier. There is no possible way for a level 3 fighter to have 13 HP it is impossible unless your CON was 4 and you had a -3 as your modifer.
Lets say we used Adventurer's league rules and you took average for HP when levelling up.
The math is like this
Level 1: 10+2=12 Total HP: 12
Level 2: 5+2=7 Total HP: 12+7=19
Level 3: 5+2=7 Total HP: 19+7=26

PHB page 12
At 1st level, your character has 1 hit die, and the die type is determined by your class. You start with hit points equal to the highest roll of that die, as indicated in your class description. (You also add your constitution modifier)

Then PHB page 15
Each time you gain a level, you gain 1 additional hit die, roll that hit die and add your constitution modifier, and add that roll to your hit point maximum. Alternatively, you can use the fixed value shown in your class entry, which is average results of die roll (rounded up).