Oh my. Well, grandmas are notorious for wanting to feed you cookies they've made or even things they've bought. Just take it as a sign she loves you very much and wants to spoil you. Grandmas love to spoil their grandkids, even when they've adults. Just say something like "I appreciate that you made/bought me *whatever the item is,* but I'm simply too full to eat it."

Please don't worry too much about four pounds. It's part of natural weight fluctuation. Many recommend only checking your weight once a week for that reason. People will get anxious about gaining a few pounds day to day, and there's a natural fluctuation. You'll be fine! Stuffing your face one day won't hurt you too much either—it's when you do it everyday that it gets to be an issue.