Hello everyone, (PLEASE READ MY STORY - I AM SO SORRY THAT IT'S LONG)

I'm come to this forum as a last resort. I've been to more doctors then I can count, have had multiple tests done and everything shows up absolutely fine.
I'm 18 years old and beginning about 4 months ago my health has deteriorated an extraordinary amount. I've done everything to try and figure out what is going on with me or to at least find the origin - but I have no answers. I'm getting to the point where I can't take it much longer, because I just feel so so weak and it genuinely feels as if I'm dying.
The best I can do is trace back feeling sick to a trip to Mexico that I took the very last week of December 2013 (it has been approximately 4 1/2 months since then). On the last day of my trip I had extreme diarrhea and was extremely nausea -I threw up multiple times. I was so sick and even flying back to Canada I was incredibly sick the whole way. I got back home and was sick for maybe another day but seemed to bounce back and was fine again - I chalked it up to eating something bad.
About a month after my trip I started feeling extremely weak, my appetite deteriorated, I developed chronic fatigue and also began to develop extreme anxiety. I went to the doctor multiple times and every time they had no answers.
It's been months and I'm slowly getting worse and worse. I truly feel as if I'm dying because I have never felt so weak or sick in my life. Here is a list of my major symptoms that are horribly affecting my daily life.

-Extreme Weakness
-No Energy
-Muscle & Limb Weakness
-Constant Nausea (About 80% of the time)
-Constipation
(when I'm not constipated I either have diarrhea or soft stool)
-Foul smelling stool
-Bad Breath
-Headaches (Pressure)
-Abdominal Pain
-Gas
-Hives
-Weight Loss
-Rapid Heart Beat
-Disturbed Sleeps (Always waking up)
-Brain Fog (can't focus, concentrate or remember things)
-Light Sensitivity (My eyes feel so weak and bothered)
-Bad Anxiety
-Numbness (all over)
.......the list truly does go on and on

Here are the tests I've had completed...
EKG - Completely normal (despite the fast heart rate)
Chest X-RAY - Normal
Blood Work - Normal
Urine Samples - Normal
CT Scan (Of Head) - Normal
Stomach X-RAY - Normal (despite seeing constipation)
Ultra-Sound (of lower abdomen) - Normal

Can anyone PLEASE PLEASE provide absolutely any insight at all to this? Does anyone think this is a parasite? Has anyone experience anything similar? I'm at the point where I absolutely can not deal with this at all and I'm so so terrified. Please help.