我的个娘？ Does this really just mean 'my mother!' and why?
Yes, you are right. 我的个娘 is just like oh my God. Chinese people don't curse their Gods, but do with their mothers, for example, 干你娘，草你妈，while these two phrases mean xxx your mother, but they also can be used to empathize the following sentence. For example, 干你奶奶的，他真有钱。xxx your grandmother, he is very rich. Obviously, the speaker doesn't necessary want to xxx your grandmother. What he want to expect is that person is super-duper rich!!! Nothing to do with grandma or xxx here. You can think 我的个娘 as "damn it". This is not really regional because other region has the same phrase like that but in other words. For example, in GuangDong and HongKong, people use 你老母，different words, same meaning. Lastly, as a foreigner, you should just use OMG because your mother is not Chinese. It makes no sense to use 你的个娘 for exclamation!!