Can you take doxycycline and azithromycin at the same time?

Well...I just did (considering that the z pack stays in your system for 5 days past the last dose...got 3 days to go)...and my stomach is hurting. Similar to gas pains. Then again, maybe it wasn't the wisest thing to do considering I have plant Allergies. I never have taken echinacea before and probably won't again.