It sounds like the card may have a problem that causes it to not report its capacity and/or free space properly. That can happen for a number of reasons, most of which aren't fixable. But if you want, you can try running a CHKDSK /x on it in a Windows computer. SD cards don't automatically remap bad sectors, so if that's the problem, CHKDSK /x will force a dismount and then check the card and attempt to fix it. Whether it works is a crap shoot, but it costs nothing to try.
Personally, I'd try another card; and if the problem doesn't recur with the new card, I'd toss the problematic one.
Richard