Thank you for the query.
If you haven't had a stool since surgery, it may be more serious than just simple constipation
Sometimes after appendix surgery, bowels obstruction due to volvulus (twisted intestine) or due other reason. Cramps and pains, lack of stool, nausea strongly indicate bowels obstruction. If there is no gas getting out as well, you may be almost sure, this is the obstruction.
and weakness can appear due to dehydration
. Back pain
seems to be due to kidneys failure and kidneys infection.
It seems that this is an urgent condition and you should not wait any longer. Please go to ER as soon as possible. Abdominal X-ray
should tell if there is any obstruction. Blood work, ions levels, CRP should be also done.
Hope this will help.