I am looking for answers to help my sister... her period is normally really heavy... and this past menstrual cycle was not normal for her she only had spotting? She is not stressed not overweight or underweight and is not on birth control... and she is not showing any signs of pregnancy can anyone give me ideas of what it could be thanks

If she has had sex, pregnancy will be a possibility that should be ruled out. It can sometimes cause spotting or bleeding in early pregnancy and many women will think it was just an off period.

It could indeed just be an off period... it happens. If there is no chance of pregnancy, she can wait until next month and see what happens with her period at that time. If still off from her norm, she can see her doctor to try and determine a cause.