Whatsapp was hacked to send message

Description

My name is Aditya Dhiman and my whats app was hacked and was used to send a message to my friend. I have an iPhone and the emoji used to send the message is not available on an iPhone so it was clearly sent from a phone with other platforms like Android or Windows. The message was sent at 10:52AM and i was away till 11:24AM when i checked my phone all my mssages had been checked by then. Luckely I detected it and have removed my whatsapp account. Request you to look in to the matter.

Please do advise what can be done to keep my self-safe and how is it possible to hack WhatsApp.