Try not to worry. I noticed my bump just suddenly seems to grow bigger, then it slows down again. If you're worried though you should probably make an appointment to see your midwife. She won't mind and it's best to be safe.
If you can feel the top of your uterus you could try measuring it yourself to see if it's large for you dates. At 21 weeks it should be between 1 and 3 cm above your belly button but at your belly button or a cm bellow is normal too.
