Whole reply history being sent with replies

Previously when I sent a reply to an email, only the new text was sent to the recipient, but recently (within the last few days) I have been having a problem where the whole of the conversation history is sent along with the new email, so that each time I send a reply to someone my emails get longer and longer as the conversation goes on. The only workaround I've found for this problem is to manually delete the rest of the replies which is inconvenient as it needs to be done every time and I often forget. Is there any way to fix this?