Femara- false + opk?

Took Femara cd 3-7 and started opks on cd 9. Cd 9 was positive, cd 10 negative... Is it possible to ovulate this early when last month on Femara, i ovulated on cd 13 (confirmed by blood test)?? I can't seem to get a definitive answer on whether the Femara can cause a false positive opk if I tested too early... Anybody had any experience with this?