If you say someone is replying to messages you send to another, how are you sure you are hacked??? A joke is much more likely. Spoof a different number and send a reply so it looks like it is from someone else. Heck it could be the person you sent the message to that is doing this. A simple app and tada the message shows up from another number.
Im getting texts from one phone number and they are actually responding to texts sent to me.
Was the text that was sent to you sent ONLY to you, or to a group of people? If sent to more than one (and you were among the group), then you "reply to all", everyone gets your response. And if they, in turn, "reply to all", then you will see their responses.
My daughter was getting texts from numbers she did not recognize when she received and responded to a text that was sent to a group. It can be confusing.