Severity of Christchurch 'aftershocks'?

We had a 7.1 earthquake on Sept 4. This morning we had a 5.1 aftershock, four months later. There is the smell of sulphur in the air and something doesn't seem right to me. Is over 4000 aftershocks 'normal' for this type of rupture? I feel seasick and physically unwell after each one. Although I live by the sea I have only started developing these symptoms in the past two weeks. What is going on?