What does fat atrophy or necrosis really mean in practical terms?

Having recently had Ulthera, and noticing some minor irregular skin changes (part of normal healing or early signs of adverse effects??) it makes me nervous. Such as could Ulthera later be proven to be damaging, like early Thermage? To allay my fears, I'd love it if MD could explain what it is, how soon or long after a procedure one would notice a change, what a person would notice superficially, how a dx is made, and is the condition permenant? Can you gain/regain facial fat (like thighs)?