It is not in your head. I had mine for 13 months. The first three or four months were hell. I felt evil, bled all the time, and was in a lot of pain. Docs kept telling me I would get used to it.
For the next few months, I did feel better. However, over time, it really started to get to me. The last three months out of the 13 were hell, too. I did not have a period for about nine to ten months and I tryly believe that the hormones built up in my body. I experienced painful joints, difficulty sleeping, my first migraine, pneumonia, extreme moodiness, fatigue, hair loss, weight gain...same as all the other ladies here.
You may get used to it, you may not. Just remember that you know your body and things do not feel right, they are not right.