happened 是从句里的动词 后面的是定语从句There are lots of changes (which/that) happened during the 1st two week in each term后面是修饰限制前面的changes的
其实这在英语中是very common的,这个动词的过去分词作定语,类似的我们可以说,There is a book wrote by Steven Huoking .
动词的过去分词做后置定语,修饰“变化changes”.