Is it my website's problem or VPS problem?
I'm assuming you mean do you need to optimise your site or need a more powerful VPS.
Otherwise I don't know what you would mean by a "problem" with the VPS. Not getting the CPU or ram advertised, unlikely. A problem with the hardware, that would affect many many customers, unlikely. A flaw in the original software configuration, again unlikely.
Where have you seen high CPU usage and is it actually affecting your sites?
You should run top (or better htop) and see which process is sing the CPU resources. It's possible you will see (most likely perl) scripts you don't recognise, in which case reimage the server and recover from backups (you do have backups?)
If there are no rogue processes then next look at your log files for errors. If there are no serious errors or unwanted access such as rogue bots spidering your site then you should consider a more powerful server.