Grainy iPhone 8 indoor photos

Several months ago I upgraded from an iphone 5s to an iPhone 8. I researched long and hard which phone took the best photos because I use my phone a lot for photography and the iPhone 8 was supposed to take the best photos in all conditions including indoor lighting. I take quite a lot of photos indoor and out and I take a lot of photos indoors of items to sell on eBay and I use the zoom quite a lot as well. I have always taken GREAT photos with my iPhone 5s. Indoor and out, and I even stopped using my good digital camera because of how good the iPhone photos were. Last night I took about 100 photos at a wedding in a nice barn with good lighting and all the photos that I zoomed in for (and not zoomed all the way in) turned out very grainy!! The ones without zoom turned out fine, all were taken with no flash. It is the first time ever that I have taken grainy photos! Sure, now and then some photos are out of focus, no biggie, but grainy...never ever! I am so disappointed! Is this because of my recent update to iOS 11.3??? HELP!!! Is anybody else suffering with grainy photos for the first time??? Any easy fix....I hope? Btw, I tried a noise reduction app on the photos and see no difference in the photo after!