Tylenol PM's active ingredients are acetaminophen (the painkiller) and diphenhydramine (the ingredient that makes you sleep). Acetaminophen is what is in regular ol' Tylenol, which is fine to take on occasion during pregnancy. Diphenhydramine, which is the active ingredient in Benadryl, is (as far as I can tell from reading) less studied than acetaminophen, but for occasional use, is generally considered to be safe. If you're just in pain, I'd just go with plain Tylenol. If you just need help sleeping, you might want to go with Unisom tablets with doxylamine, as it's been more widely studied in pregnancy, but if you need both pain relief and something to help you sleep, and it's just once or twice, Tylenol PM is probably fine.
SAH dissertating mama to my sweet little boy (2/2009)and his baby brother (7/2011)