Yes, it's fine...tylenol cold is good too...not enough of it passes through the milk...just be aware that cold medicines are very likely to dry your milk up a little. It will come back when you're done, but don't let it scare you....This is all per the pharmacist too....the first time I took anything, I pumped and dumped for 6 hours :( before I knew any better. OH well..feel better :)
Docs unfortunately tend to know next to nothing about the safety of meds and mother's milk. They believe the package inserts, which are written by lawyers trying to avoid litigation! (I stopped reading inserts after seeing a warning on my prenatals that said they were not to be taken by pregnant women.)
Kellymom.com has THE VERY BEST resource list on safe meds. Theraflu is indeed on it. SO is everything tylenol.