Q:

Why does my 4 month old playful infant suddenly start screaming at the top of her voice?

This is not a normal cry.   This cry indicates extreme sudden pain.   
For example: If you were sitting peacefully somewhere and someone threw burning coals on your body, how would you scream?
She is swaddled and fed, then she goes to sleep, peacefully for an hour or so. Then all of a sudden she starts screaming like I described above. This has started happening frequently now. 
Yesterday she was awake, happy, and gurgling merrily, and then the incident happened.
Though after some minutes she does calm down, but I am worried.
What could be the cause?

A:

You need to see your pediatrician about this. Make a sound recording and bring it along. Keep a diary. The more data you collect, the easier it will be to narrow this down. Anecdotal: one of our kids did the same thing. We never quite got to the bottom of it, but we have a strong suspicion that it was some urinal tract infection so it would hurt often when he would pee.

