Help! Positive OPK CD8??

Somebody please help, I’m super confused! I just finished my second month of Femara 5mg CD 3-7 yesterday. This morning I got a suspiciously dark opk strip so I double checked with my digital ones and sure enough there was a smiley face! Luckily my husband and I had morning sex before I even peed in a cup but I’m just wondering how long femara takes to get out of your system/ if anyone else has O’d this early?? Last cycle I O’d CD16 and before I was on Femara I O’d super late or didn’t O at all. Thanks!