Hi Sabrazee! This is exciting news, and I hope everything turns out OK! I want to let you know that I had/have a corpus luteum cyst and I started feeling it almost as soon as implantation happened. I felt it until about week 10, and even after that I had some twinges here and there. So if it is a corpus luteum issue, it's not weird to already be feeling it and you can probably expect it to be with you for quite a while!
My muscles didn't tense up like yours are, but sometimes the pain felt acute and other times it felt more achy and covered a larger area. Is it possible that you are tensing those side muscles against the pain and that's creating the muscle pain itself?
Obviously pay attention to the rest of your body, and if you feel like something is really wrong go get it checked out. But I want to at least reassure you that it is not too early to be feeling your corpus luteum if that's what it is, especially if the cyst is very large, and it probably won't go away until the placenta takes over. Isn't this fun?