Undiagnosed for nearly 4 years: nausea and upper abdominal pain, weak

Hi there,
I was a fit and healthy 19 year old, when I was struck with immense nausea and upper abdominal pain, bloating and extreme fatigue to the point where I'm bed bound, and weak legs, which I can only describe as feeling as though they've been filled with concrete.

As the years have progressed, I now have heat intolerance, lesions on my scalp, my heart rate goes up over 40 beats per minute when I go from seated to standing, and yellow bruises that seem to appear on my legs for no apparent reason, and I often wake in the night with a knawing pain in my stomach like I've never eaten before.

I have seen a gastroenterologists who have ruled out crohns disease, IBS, gastroparesis, inflammation in the gut and food intolerances and have discharged me, saying there's nothing more they can do for me.

I'm at my wit's end, and have spent almost four years bed bound. No anti nausea medication helps at all, and the only medication I'm on is sleeping tablets to sleep through the pain at night, although it still wakes me during the night.
I have attached pics of the scalp lesions.
If anyone has any clue as to what this could be I would really appreciate some help as I'll be turning 23 this year, fell ill when I was 19 and just want some kind of quality of life back. Thank you so much, in advance!