my mother had an ileostomy 4 weeks ago due to the bowl being gangrene, and is now suffering from extreme tiredness and feeling

Undergoing any bowel surgery is a major ordeal.  The longer she was in bed, the longer the rehabilitation.  We typically estimate 7 days of rehab for every day spent in bed.  Since this was just 4 weeks ago, I suspect she's still very much in the healing phase where her body is using all its energy to recover.

Is she eating enough & drinking plenty of fluids?  Any new medications to her regimen that might be interacting w/other ones?  Any fever, sweats or chills to make me think of infection?   

With that said, she should be followed closely during this post-operative stage by both the surgeon and her family physician.  Chief among common causes of feeling extremely tired is anemia due to loss of blood.   How's her breathing?  Often times the lungs don't open properly and she can have atelectasis.  Then she can't get enough oxygen which will make her tired.  

I suppose she could have had borderline hypothyroidism which was aggravated by the surgery.  Or perhaps she's in a subclinical Addisonian crisis and needs more corticosteroids?  

Best advice, as always, is to follow up with her surgeon & family physician.  Tell them what you've noted above.  Good luck!