WHY GonalF and Femara??

I am very confused. I have taken letrozole and produced 2 very mature follicles each month the last 2 months. So 4 eggs, TI, no pregnancy. I have PCOS and am on Metformin. My RE stated if we wanted we could do Letrozole and GonalF (small dose) and our 1st iui this month. She explained that due to age (34) and still trying to have our first baby that injectibles help increase success rates. I agreed and my meds are ordered, but I realized later that GonalF does the SAME THING as Letrozole. We obviously don't want more than 2 eggs because that could lead to a cancelled round of IUI. So why on earth would she suggest GonalF if I was already responding to Letrozole well the last 2 months?? I'm nervous, will I likely over stim? I don't have an ultrasound until Monday.