When do you count your start date after MC?

Ashley • Expecting twins May 2016!
When should your 'period day' be counted and when would your fertile week be after a mc?
​A little background info on my situation: 
​I had a miscarriage/chemical this past week. Doctors classified it mc but I had a positive and started bleeding 3 days later so I lean towards calling it a chemical. (Went to doctor to rule out ectopic when I started bleeding and they found hcg levels on Wed but none on Friday.) 
​Anyways, I've had a few friends say you're very fertile right after a mc and we want to try again right away. The doctor said that would be ok but she wants me to see a specialist because of my past history. (Had to terminate at 23 weeks due to a fatal fetal anomaly called limb body wall 4 years ago.) So I schedule that this week and can ask then as well.
​Back to the original question, when would you think I should start testing with opks?
​ 🙏👉👶