Hello,
PCOS is a common disorder in women of reproductive age, affecting 5–10% of premenopausal women.Women with this syndrome may present with menstrual irregularities, chronic anovulation, infertility, obesity and hyperandrogenism. Insulin resistance and compensatory hyperinsulinaemia are prominent features of PCOS .

Metformin won't restore ovulation in all people. it is more likely to do so in those that are overweight, as they have more significant insulin resistance. Long term use can also lower androgen levels which can help, but full effect can take 6-8 weeks. 
In general, in a lean PCOS patient, it is usually less likely to restore ovulation. If you are trying to conceive, you need to consider other alternatives than just metformin alone.

Since you are lean and slim,I have my doubts regarding the use of metformin in you.
You should discuss with your doctor regarding this.

Metformin  reduces insulin resistance of peripheral tissue and allows muscle and adipose cells to utilize glucose at normal insulin levels. 
Since type 2 diabetes associated with overweight and diabetes and PCOS patients have marked insulin resistance,this drug is commonly used in both conditions.
Thanks

OBJECTIVE:
To assess pregnancy outcome in anovulatory infertility patients diagnosed with polycystic ovary syndrome (PCOS) who were treated with metformin.
DESIGN:
Case series.
SETTING:
Outpatient.
PATIENT(S):
Anovulatory patients (n = 48) with a diagnosis of PCOS based on clinical, diagnostic, and laboratory evaluations were enrolled in the study over a 15-month period.
INTERVENTION(S):
Metformin was started at 500 mg b.i.d. for 6 weeks and then increased to 500 mg t.i.d. if no ovulation occurred. Clomiphene citrate (CC; 50 mg) was added if no ovulatory response occurred after 6 weeks.
MAIN OUTCOME MEASURE(S):
Resumption of menses, presumptive ovulation, and pregnancy.
RESULT(S):
Nineteen of 48 (40%) patients resumed spontaneous menses following treatment and showed presumptive evidence of ovulation with metformin alone; 15/48 (31%) required CC (50 mg) in conjunction with metformin therapy, and 10 of these 15 (67%) had evidence of ovulation; 20/48 (42%) conceived with a median time to conception of 3 months, and 7 of these 20 (35%) had spontaneous abortions (SAB); 19/48 (40%) had gastrointestinal-related side effects, and 5 of 48 patients (10%) had to decrease the dosage of metformin. Only 1 patient discontinued therapy.
CONCLUSION(S):
Metformin alone in patients with PCOS results in a substantial number of pregnancies, with 69% (20/29) of those who ovulated conceiving in less than 6 months.

Hi friend,
Welcome to Health Care Magic

Check more and you will find!
One medicine can be used in several diseases / several medicines can be used in one disease.
Metformin has several actions - insulin resistance, androgens. lipids and so on, which help in PCOS.

Take care
Wishing speedy recovery
God bless
Good luck