Side effects of Iron supplements

<span>I had 90% of my colon removed 3 months ago.  I have ulcerative colitis.  I have been extremely fatigued and weak.  I started taking Iron 3 weeks ago.  I started with one pill a day (160mg of Ferrous Sulfate:  The product is "SLOW FE".).  After 3 days I took 2 pills a day.  Now I am taking 3 pills a day.  I am getting stronger and I am not as tired.  I am concerned about overdosing.  What are the symptoms?</span>