If the doctor says you have very little brain activity how do you have seizures?

There is constant electrical activity in everyone's brain at all times. When a seizure happens it is because there is a sudden increase in electrical activity, like a storm or power surge. That soon passes and electrical activity returns to normal and the seizure ends. So if when someone with epilepsy is examined at a time they are not having a seizure, then their brain's electrical activity will appear to be normal.

To test for epilepsy what is sometimes done is to expose the brain to known triggers for seizures to see if there is a change in the electrical activity so that doctors can find out what causes an particular person to have seizures. Different people will have different causes for their seizures. So if the right trigger is not used during an examination, the person will still have normal brain activity. That does not mean that they don't have epilepsy and won't get seizures. It just may mean that their particular trigger wasn't one of those that were tried on them. As there are many of them, it is not possible to test for them all in one examination.