I had roughly 17000 on my phone before it had to be sent for repair.
Are you sure your sim isn't limited to 3000 per month?
Usually if it's unlimited the limit is 3000 and it won't let you send more. Perhaps by deleting some you trick the system or something?

Try checking your bill details online or ring up your network provider before faulting the phone. Might even just be a dodgey sim card!