Oh boy! looking for advice

Hey ladies! Just found out last night that I'm pregnant. Here's the problem...I have NO IDEA when my last period was. I'm so busy chasing around my 16 month old, and my husband and I weren't planning on trying to conceive for a few more months. I'm also on an antibiotic which is a category D drug (very detrimental to fetus) and have been for 13 days. Clearly, I stopped taking it last night. I'm also on Zoloft for PPD from my first. I need any insight you can give me on how I know when I'm due and safety with medications. TIA!