from your history and complain, it seems that you are suffering from chronic dysentry
.you may consult your doctor,one course of anti dysentry and some antispasmodic medicine will solve your problem.
as you are thin and slim,your digestion is affected due to chronic dysentry. if one course of anti dysentry gives you relief,after some time go for another course,your digestion will improve and you will get good appetite and will gain weight.
mean while you take light diet,more liquids,lemon juice,butter milk and curd