My Boyfriend is Getting Texts from Me That I Am Not Sending

My Boyfriend is Getting Texts from Me That I Am Not Sending

When texting with a certain person, he receives texts from me which I am not typing. They appear, but then do not stay in the thread. He is in San Diego and uses a Google Nexus 5X with Google Project Fi service, and I am in San Antonio, using a Samsung Galaxy J3 with Virgin Mobile service. It seems to only happen when his screen is closed. He sees the message come in, but when he opens to the thread, it isn't there. Could they be previous messages that are coming off our thread as we run out of space?