email reply of nasty pictures from unknown when replying someones email

[sam11]

Hi everyone. I have a problem.
When I send reply to a certain email address; somehow my reply is forwarded to emails of some girls in porn site or whatever. then she replies with nasty pictures that she wants to hook up or something. I understood that it happens only when I reply a certain person by looking at the subject of the email. Its the same subject as the thread started by that certain person and only that person. That means that some malware is on his computer, right? Not on mine? Kind of scared right now......its getting really annoying. Please help!

 

[spentshells]

Well what is the email address? Does it belong to a friend? If so the email password and contacts list have been stolen.