What are the statistical chances of pregnancy?

I had my Mirena removed a month ago. I was on my period. about 15 days after the first day of my period after mirena was removed me and my husband had . that was about 16- 20 days ago. I am having alot of the same things i had with my daughter early on. bad headache wont go away, nausea, weak feeling and very tired. i took test it was negative but the pamphlet and everyone else say its to early for that! what i want to know is how early should symptoms come and when should i test effectively?