Thanks for your query.Based on the facts that you have posted it appears that you had unprotected sex with your boy friend after your last period on 18 th March.
If your menstrual cycle is regular your period was expected on 16 th April ,
You have noticed slight spotting today.and suspecting to be pregnant.
Since you have unprotected sex possibility of pregnancy
can b b not be ruled out.
Please wait for few more days and get your urine test for pregnancy done. If it turns out to be positive consult qualified Gynaecologist for examination and further treatment .