Why is my brain so foggy these days?

Stress and fatigue. Given that you are only 23, this is most likely caused from a lack of sleep and stress. If you have any signs of stroke or weakness in a part of your body, you should see a doctor, but most likely you need to get some rest and start reducing stress.
Brain fog. Brain fog is a real symptom that can have many causes. Sometimes it is due to illness or hormone fluctuations commonly seen in women in the years before and during menopause. Mood disorders such as depression can cause this symptom. Keep a journal concerning your activities , what you eat and the brain fog days and see if there is a pattern.