I'm 19 weeks pregnant and took robotussin dm about 4 hours ago. Can I take benadryl (diphenhydramine) to help me sleep so I can actually get some decent sleep?

Which Robitussin DM (guaifenesin and dextromethorphan)? There is one for night time, which contains antihistamine, doxylamine, so Benadryl (diphenhydramine) then would be an extra dose, you need to be careful. Or the other Robitussin max strength DM, which doesn't contain an antihistaminic, then it shouldn't be a problem with the antihistaminic, read the labels carefully, also read about warnings with pregnancy, good luck.