Viktoria Tautz, 34, was looking after Joshua Paul at his home in Haringey, north London, on 29 August 2014 when he suddenly collapsed.
Joshua was taken to hospital but died on 1 September, the Old Bailey heard.
Ms Tautz, of Holly Park Road, Barnet, denies one charge of manslaughter and insists she never got angry with him.
The court heard Joshua, who was born about 10 weeks early, had a large head for his age and was being monitored.
Asked what she knew about his condition, Ms Tautz said she was aware his head was "big" and "sensitive" on the top.
After starting work for his family in June, she said she was getting on well with the parents.
On Joshua, she said: "He was good baby."
Bernard Richmond QC, defending, asked how she coped on days when she found him in an unhappy mood.
Ms Tautz said: "There was not much needed. I lifted him a bit. I played with him. He calmed easily."
The barrister asked if she ever got "angry" or "frustrated" with the baby.
She replied: "No, never."
On the day of Joshua's death, Ms Tautz said he was lying on his back making "baby noises" so she talked to him in English, thinking he was not tired enough to sleep.
When he began "crying very intensively" she decided to pick him up to "make him calm down", she said.
"When I picked him up his face was red. I recognised his face seemed swollen," she told the court.
Joshua did not appear to be breathing and she "got scared" - taking him to the bathroom to try and induce vomiting, the jury heard.
"Everything happened very fast", she said.
"I was absolutely scared. I didn't know what happened to him."
Ms Tautz was arrested on 5 September and in police interviews said she had not had any accidents and denied shaking him.
Jurors were told Joshua died "because of a head injury that caused bleeding in his brain, eyes and other spinal injuries".
The trial continues.

The first leg of the Champions League qualifier at Windsor Park in Belfast was marred by incidents.
Play was held up when a bottle was thrown at Griffiths and there was an attempted pitch invasion at full-time.
Linfield have been charged by Uefa with the "throwing of objects" and a "field invasion".
Scottish champions Celtic have also been charged with "improper conduct of the team" after they picked up more than five bookings.
One of the players booked was Griffiths, who was shown a yellow card by referee Alejandro Hernandez as he picked up a bottle that had been thrown at him while he was taking a corner.
Police with dogs were deployed at full-time to stop a pitch invasion.
European football's governing body Uefa will deal with the case on 20 July, the day after the return leg in Glasgow.
Celtic had decided not to take up an allocation of tickets for the game because of security concerns - but around 300 fans watched.
Speaking after the match, Celtic manager Brendan Rogers said: "Players go onto the field and it should be a safe environment for them. You want the passion from the stands to stay in there - but if you take that away from it, it was a good evening."
Police Scotland released a statement urging fans to remain "focused on the football" during the return leg.
Superintendent Craig Smith said a "robust policing plan" had been put in place following co-operation with the Police Service of Northern Ireland.
He added: "Anyone who is intent in causing disorder or hate crime can expect to be arrested by police."
A nanny accused of leaving a 10-month-old boy fatally injured in a fit of anger has denied to a court that she lost her temper with him.
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Celtic striker Leigh Griffiths has been charged with "provoking spectators" after he tied a scarf to the goal following Friday's 2-0 win at Linfield.