Hit points are determined by the size of a creature through their Hit Dice, and the Constitution modifier of the creature. You add your Constitution modifier to each Hit Die you roll for your hit points.
If your Constitution modifier changes, your hit point maximum changes as well, retroactively.
Temporary Hit Points
These are extra hit points gained through spells, equipment or feats, often with a limited timespan. You may go beyond your maximum hit points when gaining temporary hit points and damage suffered is first subtracted from your temporary hit points before taken from your regular hit points. It's recommendad to write down your temporary hit points seperately from your regular hit points.