Rectal bleeding only on period.

I don't think it is blood from your rectum unless you have ruptured a hemorrhoid or something obviously painful.  During your heaviest days of bleeding you can lose enough blood to make the toilet look reddish but its coming from your vagina.  As, far as not being able to go number 2 … do you use tampons?  If so take it out before you go to the bathroom and see if that helps.  its sounds like you have heavy and painful periods which are not uncommon you should tell you doctor because they can do things like put you on birth control.  I had heavy painful periods and was on birth control by the time i was 12 and then my periods were 2 days of light bleeding and no cramps you don't have to be sexually active or even get a pelvic exam if it makes you uncomfortable they will just write you a prescription until you become sexually active or 18 whichever comes first.  Don't be embarrassed you are going to have you period once a month for possible the next 40 years, same as every women on this planet.  your mom will understand especially if you are missing school.  Definitely see a doctor though because if there is something wrong you NEED to know.  Make sure to always lister to your body and take care of your lady parts because one day you want to have children of your own.  You have a lot of options when it comes to birth control too, search it on google, anything from the birth control pill to intrauterine devices.  A lot of changes are going to happen to you over the next few years as you become a young woman and embrace it we have all gone through it never be ashamed to be a women!