Help please! Baby seems in pain and I don’t know why

My baby’s almost 4 months old and he’s usually very chilled and doesn’t cry much. Only when he wants his dummy or is hungry. But this afternoon he’s been screaming on and off.

He suddenly starts crying, goes red in the face and cry’s so much he stops breathing for a minute and goes silent and cries out again. I’ve stripped him off, checked his body all over. And I can’t find the cause.

Someone said gums but I’ve checked his gums and he doesn’t seem bothered when I touch them and they don’t look red or anything.

Any ideas as to what I should do if this carries on tomorrow? He’s feeding fine and his temp is ok

Advise please!!! X