22 year old woman. colon cancer possibility?

Im a 22 year old woman and for the past few weeks, i have had a horrible scare about colon cancer. I've had very irregular bowl movements. One day I may be constipated and another i may have diarrhea. I sometimes have a pain in my anus. For instance, when i squeeze my buttchecks together, i feel a sharp pain. The pain doesn't necessarly hurt, but is more like a pinch like feeling. I haven't noticed any sort of blood in my bowel movement, but one a few occasions, i have noticed some clear mucus. Also, i have noticed that some of the skin around my anus is somewhat "loose" (not sure if thats the right word to use). Im guessing that can be the result of a previous hemmorrhoid? not sure..
My grandfather was diagnosed and treated for colon cancer a few years ago. He was in his late 70's when he was diagnosed. The doctor didnt mention anything about the hereditary colon cancer (FAP).  
I'm scared. I don't really know what to do? Should i go for a colonoscopy?
any feedback will be veryyy appreciated. thanks in advance for the help.