I began taking an antibiotic for cold/sinus symptoms. it's a z pack and my stool has been greenish and runny. is this from the antibiotics?

Could be.: There is an antibiotic-associated diarrhea which can happen after only a very short course of antibiotics. Stopping the antibiotic is often enough to stop the diarrhea. If it's voluminous, frequent and watery, by contrast, it could be another infection,,but you'd typically need more than a ZPack worth of antibiotic to cause this. Wouldn't be green and runny, more watery.