I also get my own message when I reply to a group text, but I don't think it was happening before the upgrade to 8.1. I'm just running whatever AT&T pushed, 8.1 and Cyan, nothing pre-release.
Sometimes I can't send/reply to group messages at all. I get a vague message saying the message can't be sent and to try again, but no amount of retrying works. The other day I could reply to a group message that consisted of two other people, but not to another that consisted of 11 other people.
I don't know if it's a problem with one or more of the recipients, or the whole group messaging mechanism.