10 days ago I started doing exercises. Now, I don't have much of an appetite and I get a weird feeling in the pit of my stomach. Also, insomnia?

Overtraining? What type of exercise, how often, and what duration/intensity? Exercise is associated with posiitive outcomes, however, sudden, rapid increases in highly intense exercise may produce some of the symptoms you are experiencing. Exercising at night may also lead to difficulty sleeping. Recommend gradual onset of daytime exercise to see if this helps. If not please have an physical exam with doctor.