What do you do when a baby won't stop crying/screaming and there doesn't seem to be anything you can do?

Watching my 4 month old grand baby. At one point, right after a nap, she started crying and crying. Changed her diaper, which wasn’t dirty or wet. She was happy as long as she was nakki and I was talking to her, but after that she started crying again.

Tried taking clothes off in case she was hot, then putting clothes on again. No change.

Tried cooing and talking to her. She screamed in my face.

Trying to feed her just made her angrier.

Walking around with her slung over my arm helped, but after 10 minutes my back started hurting. The the instant I sat down, even though she was still slung over my arm, she started screaming again.

After an hour and a half I finally moved her playpen into the bedroom, laid her down and shut the door.

She cried for about 15 more minutes then went to sleep, probably exhausted.

Man. Haven’t had to deal with that in 28 years, when my middle daughter did that. Nothing seemed to help.

What’s going on when babies do that? It is so sad, and it’s an awful, helpless feeling not to be able to help. :(