Low memory warning iPhone 4?

I've had my iPhone for a while now, and up until a couple days ago it was great. Then all of a sudden, my download speeds dropped and I started getting a low memory warning. When I go to the settings (under usage, which is where it tells me to go) it says I still have 3.8gb left. I've tried turning it off and back on, I've tried factory resets (restoring from iCloud so as to not lose my game info), I've tried force closing all the apps in case it meant RAM. Nothing seems to work. Is there anything I can do to fix it, or will I have to send it in?

Reset settings