Why have some people gotten sick after getting the pfizer vaccine?

Some people's immune systems cause more symptoms when they respond to the vaccine. It appears to be random; maybe some had prior coronavirus colds. Don't over-worry about media reports: the large majority of vaccinated persons have only mild symptoms or none at all, either to the Pfizer or Moderna vaccine. Symptoms serious enough to limit activity for 2-3 days probably occur in under 1%.

Side effects can occur with any vaccine - not just the Pfizer COVID -19 vaccine. The symptoms show that your immune system is responding to the vaccine. The symptoms are usually short-lived and tend to completely resolve.