Yes, worry. Get in to see your Eye MD tomorrow. It may just be a posterior vitreous detachment but having had cataract surgery you are at an increased risk of retinal detachment.
Use the searach feature and read about PVD, and RD.
JCHI IIMD
Anytime you see a new large floater (and especially flashing lights) it's important to get an exam by an optomologist. There is risk (low) of retinal detachement with those symptoms, but you need to get it checked out. find a doctor at aao.org if you need one.