My chocolate-chip cookie expanded after baking
There are many reasons why this can be happening, cookies will spread more if the oven is not at the right temperature, try preheating for 20 minutes before you put cookies in the oven. Our ovens sometimes change through time and it might not be  hot enough. Another thing could be that you added more sugar than before, sugar tends to make the cookies also spread if you add more that you were supposed too. Another one could be, since air will cause cookies to spread, you don't want to whip too much air into the cookie dough when you're creaming the butter and sugar together. Only cream for as long as it takes to combine the butter and sugar, which might only be 30 seconds or so. Beyond that and you're just incorporating too much air. SO check on this things and maybe something here rings a bell, this has happened to me when I have substituted margarine for butter, and sometimes yeah I tend to cream a bit long cause I get distracted. So yup check on this things :)