Thank you so much for your replies and advice! You have given me a lot to think about. You guys are right. I guess my main concern was that if I didn't follow the rotation as suggested, or if I added more cardio, that it wouldn't be as effective or that I wouldn't get the same results. I was much more worried about that than how i physically feel, when I should really start listening to my body and if I feel weak, sore or tired i should just ease up a bit. When I started the rotation my plan was to add some type of cardio. I have also been doing some short Leslie and Jessica Smith workouts in addition to slim in 6, but I just wasn't sure of how I should incorporate them in this specific rotation. Maybe that was the problem. I wanted to do too much and i was overdoing it. Now I will definitely combine Sin6 with cardio. That would help keep me motivated as I absolutely love cardio and also will help while my muscles get stronger and used to working out again.
I also should definitely add some yoga and/or stretching too, like some of you said. I have to admit that I'm not really a fan and sometimes I even skip it, but especially now that I'm restarting stretching would benefit me even more to let my muscles feel better.
Thank you again. I knew I could count on you to help me out. And now that i'm not lurking anymore, i hope to post more and to get to know you guys better, and that you can get to know me too!