Craving cigarette

The last few weeks of my pregnancy, and now almost 3 weeks pp I've been craving a cigarette so bad. Especially after the day I've had today. I use to smoke, but I haven't in almost 3 years. I'm worried I'm about to cave in. I do not want to smoke. I don't want to smell like it, I don't want to he unhealthy, I don't want my son to smell it on me or anything. I'm just forcing myself not to buy a pack but it's getting hard. And This just came out of no where.