Really important question

I know I'm not pregnant got blood work done and my hcg is less then .1 so that's not the case. Well my AF is now 5 days late. My Dr put in me for to start my 3rd cycle of clomid and instead of doing 50mg he upped to 100mg but my AF still isn't here. I have checked online if I'm able to take it without AF and I see so far the only reason they recommend you have AF when you start is encase your pregnant. But my question is if I know 100% that I'm not pregnant can I start clomid.