It is very unsafe for you to take your mothers diet pills.
The pills were designed for overweight adults and will make you very sick.
5'4 and 115 lbs would make you 5 lbs underweight NOT overweight.
At 5'4 the least you should weigh is 120 lbs.
I'm 5'4 with medium frame and my doctor told me I should weigh no less than 125 lbs.
The avg weight for a 5'4 female, depending on body frame, should be between 120 lbs and (142 I think) lbs.
I think you're being too hard on yourself...