Text tone rings but there´s no text message?

My iPhone keeps making the text messages sound, as if I got a text message or an WhastApp message, but there is no message.

Why is this?

This has happened on several occasions. And it makes several text messages sounds in a short period of time, as if I have received several text messages.

But there are no messages. Or any other notifications. I don´t know what it is?

Does anyone know??