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 :)
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.