Yeah, it sounds like the chain is too far off towards the smaller cog.
But "suddenly" sounds unusual. Was the chain kind of noisy from slight gear grinding previously? (an indication of being out of adjustment for a while.)

How old is the cassette? This popping out of a cog can happen with a new chain on a worn cog. On a hard pedaling effort, the chain rides up on the worn teeth, then jumps off to the next cog.

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To check the rear derailleur: (You probably know all this, but it may help new riders.)

In the big chainring, shift all the way to the smallest cog. Then shift 3 more cogs. (Just to make sure it's a clean / normal chain position.)

Get off the bike. From the back, look at the chain position between the cogs on both sides of the current cog.
You want a very slight gap between the chain and the next largest cog. As small a gap as you can make it.

If you turn the adjuster counter clockwise to move the housing outward from the derailleur, it effectively lengthens the cable wire, pulling the derailleur inwards, toward the next larger cog.

Do a small adjustment, maybe 1/8 turn. Then repeat the shifts to the smallest cog and back 3 cogs larger, and check it again. I'll finish with very tiny adjustments to get it "just right"

This process is easier if you can support the rear wheel off the ground on a repair stand. Or have someone hold the saddle while you shift and adjust. But you can always do the 1/8 turn, get on the bike to do the shifts, then get off and look again.