Terrible cigarette smell entering my apartment?

Gabriela • First time mom. 23. Due 06.26.17
I'm 20 weeks pregnant and I'm freaking out because there is a terrible cigarette smell that comes once in awhile every night. 
I don't know if it is because the previous tenant use to smoke. But that was years ago, I doubt that smell would be coming now. 
Or is it coming through the heater vent from someone else's apartment. If that even is possible, because it's not one in the ceiling it's a big ass heater on the floor. 
What can I do to eliminate this horrible smell? 
Thanks!