Unless you ovulate early, it's unlikely you are going to conceive on cd4. You need to be having sex mid cycle....around the time of ovulation to conceive a baby. If you don't get your period by next week, then take another test and if it's negative then it's not likely you're pregnant.
well, you should have sex around the time of your ovulation. i dont think there is a chance to get prego. n u said ur baby is 18months so i think its normal to hv breast milk this time. (shhhh my mom was breast feeding me till i get 3 yer old sshhhhhh). so i guess u shouldnt b worried about this. and if u r TTC then track ur ovulation. Good Luck :)