I Am A Vegan Due To Allergies, I Am Not

I am a vegan due to allergies, I am not eating properly, due to flair ups.
Suffer with Crohns, osteoporosis and been monitored for my heart (dizziness and weakness)
I am going to have a colonoscopy on Sunday AM at the moment my diet is very poor.
I am a good cook, but have been suffering with glandular type fever since on and off November so
very fatigued all the time. I would be grateful for any assistance regarding what foods I should have for the process.