blood test still negative ?

so i had unprected sex the days that are cirlcled including the 10 also that i forgot to circle i had a doc. appt . which they did a blood test as a pregnancy test and it came out negative for pregnancy could it still be too early what day would i have conceived ? or if i did . they want me to go back on the 15 of march and 11th of april . im confused by the negative result . ttc is there still a good possabliity that i can still be pregnant ?