Brightside deletes my messages!

[16aotto]

Okay ive had my phone for a while and have seen that when my phone is running low on memory for my txt messages, it starts to delete the messages i sent to people from old to new. if i don't agree with that happening i delete some messages and everything goes back to normal. However earlier today i noticed it happening even though i only have a few messaged in my inbox thing.  almost directly after i send a message to someone my phone deletes it; not the received messages only the sent ones.  I dont even have time to lock them if i want to keep the conversation. what do i do!!!!!!!!!