Geetings Nickelkriss, you are not being too presumptious posting on this site, you came to the right place.
First of all do not rule out Crohn's, because I am 60 and most of my life until about 2005 I had severe constipation. I used to be able to go a week without a bowel movement then I was diagnosed with Crohn's in 2007.
I think that you should do what your doctor said and take the proper preparations for your test because you really need to find out what is going on.
Your list of symptoms could be Crohn's so you should really get checked out to be sure, and if it isn't Crohn's your doctor can help you find out what you could have. I always say it is always better to be "safe than sorry".
I am so sorry that you are feeling so bad and hopefully the people on this site can help you, they are not judgemental, so if you need to ask more questions, go right ahead.
We are not doctors but one thing we are is supportive and caring.
Good Luck to you Nickelkriss and let us know how things turn out.