I took the stuff that makes you drowsy whilst bf, because i came out in an all over body rash a few days after delivering ds. Had to stop taking it in the end because i was really struggling to do night feeds, and felt like a zombie in the day.
i took drowsy piriton as a sleeping tablet when about 7 / 8 months pregnant and while breastfeeding, but only after the evening feed. dd2 was old enough to sleep through til the morning so i don't think she got very much of it. didn't affect her poor sleeping anyway!