What was strange was that I was on a train at the time out of WiFi range, and I don't store my Skype password on my phone. So it's not like there was a device w

What was strange was that I was on a train at the time out of WiFi range, and I don't store my Skype password on my phone. So it's not like there was a device with malware sending messages through Skype from my laptop as it couldn't reach the internet. It's possible I had e.g. a keylogger but that wouldn't explain why only Skype was compromised.