Involuntary typing of the letter 'Z'

I have this MacBook Air since 2016. From last evening I'm noticing that the letter 'Z' is getting typed in chat boxes, email compose window, word documents, excel cells... In short, in all the places where there is an option to type anything. Today morning I saw after 'Z', there is sometimes a drop-down menu showing z with a dot on top of it with 1,2,3 below them. I'm sorry I don't know the exact terms for them. I can't even start my laptop now as the password space is also getting covered with Zs. Can someone help me please?!