Blood test for constipation/colon cancer?

If I go to the doctor can he tell if I have constipation/Colon Cancer by tacking my blood? I have been feeling a little constipated lately and have not been having a good bowl movement lately either.I also noticed that I have been feeling gassy too,I also have I think an anal fissure and was wondering if putting rubbing alcohol on it would hurt anything? I soak a tissue with alcohol and kind of rub it on there.It hurts but slowly goes away each time I rub. I need some help BAD! I hope I haven't got something serious wrong with me. I mostly got gassy feelings and I also got bleeding but little spots but I think it's from the anal fissure.I want to know what people think could be wrong with me. Do I got constipation or colon cancer? I am hoping it's constipation cause it's easy fix,I used to eat those chocolate laxatives but quit cause they say it makes the mussels in your colon weak or something. Like it will make your colon depend on them for having a bowl movement,so does anybody have any advice before I see the doctor?