My iPhone 3GS is automatically deleting text messages? - 1

I've had my iPhone for a few years, and it's never done this before. I have a conversation with one person that dates back to last summer. We've exchanged about 150 texts, and none of them have been deleted. But tonight, every time I sent a text, the oldest one would be automatically deleted. How can I fix this? (I jailbroke my phone 4 days ago, but I've texted this person twice since then, and nothing has been deleted before tonight)

Phone memory is full. Delete a bunch of messages and your problem will go away.