Car is jerking, what could cause this?

So my car recently started jerking while I’m at a complete
stop and sometimes while driving. It’s not a heavy jerk but
it’s enoufh that I can feel it. My engine light is not on but I
have been told I might need new brakes and a belt soon.
It seems to be worse when I first start driving then it
almost goes away completely after about 30-40min of
driving.