Why am I getting a low memory warning on my LG Octane phone?

I have a 16 GB miniSD card on my LG octane. All my music is on the card, not on the phone's memory. When I check the phone's memory, it says I'm using less than a third of the phone's memory, that more than 2/3 is available. But I keep getting a "memory low" warning that asks to delete all my text messages. All of the messages have been deleted, so there should be room. Just earlier today I had all my messages on there, and I haven't changed anything since then, but I keep getting the same notification. Anyone know how to solve this?