Email Message Changed to Random Characters

I have a @hotmail.com email address and my friend has a @gmail.com address. We were exchanging emails and all was fine until my latest reply. She wrote back, "I don't know if you tried to send me something, because all I got was a ton of random characters." Lo and behold, below her reply, the email I wrote was nowhere to be seen; a huge paragraph of random letters took its place! I checked my Sent Folder in which I saw the email I wrote was displayed exactly as I wrote it -- no random characters. I thought maybe the unusual characters I added (two hearts written by Alt + 3) in the email might have messed it up, but I think that is a long shot. 

Also, I noticed something a little unusual in her reply notifying me of the random gibberish. It said "*** Email address is removed for privacy *** on behalf of Her Name (*** Email address is removed for privacy ***)". I'm mentioning this in case it is relevant.

I have sent other emails to Yahoo addresses, to Hotmail addresses, and another Gmail address, and all of them were sent normally -- absolutely nothing odd. I even used the same browser: Google Chrome. Is this problem on my end or my friend's? I have screenshots to provide if needed.

My password is 14 characters long and a mix of capital letters, lowercase letters, and numbers; I don't think I my account was compromised.