why does my poop float and why is it black?

well I am 22 and my stomach has been giving me major issues for about 3 yrs now!! it started when I was a heavy drinker and it was then a urianry tract infection and kidney infection I toke care of that, sort of, I could not afford the 240 dollar meds they ask me to take so I was not able to take care of it fully. I was not able to drink like I used to after that. I also changed my diet at that time to much better food, maybe having a beer or 2 at a meal (or wine) I was able to get it feeling alright after that but now it has been about 2 yrs and I now stuck with a different problem, my poop floats!! sometimes if I eat the wrong thing I dont poop. I try to eat just greens, lean meats, and fruits sometimes I go a stray and eat junk food! I feel everytime I do eat junk food I am nauseas,fatigue,well I pretty much dont feel good for a couple of days until I can flush whatever I ate out!! my boyfriend says I say I am nauseas after pretty much everything I eat but I feel it is from all the c**p they put in junk food! I am tring to stay away from these things cuz some one told me maybe I cant process fat!??! I cant find anything on this so I was woundering if you could help me!!?? I cant afford a doctor bill at this moment if I could just find out what it might be I can elemenate options so when I do go to the doctor I wont be wasting money on a bunch of random test......I heard sometimes they dont find what the problem is for yrs..... %-)