Quitting smoking while pregnant...

I quit smoking cigarettes a week ago at 29 weeks (pls no hate) it was hard to do but I've managed. Well my bf just had a cigarette and I took three drags from it.. I don't know why. I didn't want to but I did. I'm just wondering if this is really bad, quitting then suddenly smoking and quitting again?I know smoking is bad in general just want to know if smoking after you quit and then stopping again is bad