Unexplained rapid weight gain, fatigue, anxiety, depression, hair loss and unhealthy, muscle weakness, no stamina, loss of interest in everything, flat affect, swollen left side of neck.

What could be wrong with me? Should I see a doctor? See above listed symptoms

It is kind of a self explanatory answer in my opinion. You have a whole list of wide ranging symptoms. They are bothersome to you. I think you already know the answer to this.

Go see your doctor.

Yes, see a doctor as soon as possible.