My sis is having fever every month. she had slimming pills for some time. do u think it has any relationship with this freq occurrance of fever?

Probably not, but...: Fevers "every month" need to be evaluated. They may not be related to medications, although sometimes they are. A patient can see her doctor, who can do an evaluation and order some blood tests. Then the patient can write down her temperature each day for several months, and take the calendar of data to the doctor for each follow-up visit to see if there is a disease.

How high: A women has regular temperature variation with the menstrual cycle and temperature goes up in the second half of the cycle. What she perceives to be fever, may be physiological.