Yes, canned tomato products can be high in sulfites. The boiling water used for canning often contains sulfites to prevent rusting of the food processing equipment.
Also, tomato skins themselves can contain naturally occurring sulfites.
I used to be able to eat peeled, fresh tomatoes but not canned. Now, my sulfite sensitivity has gotten so bad I cannot eat tomatoes of any kind anymore.
It all depends on your level of sensitivity what foods are tolerated vs. not.
Re: reaction from Epsom salts - may also be a detox reaction, i.e., accumulated toxins exiting the body, facilitated by the Epsom salt soak. I don't tolerate SLS in shampoos, etc., but am ok with Epsom salt soaks as long as I don't do them too often.
Edited to add:
Yes, UC particularly can have a connection to sulfites...watch out for animal proteins.
Also there are bacteria that thrive on sulfites, that may cause inflammation when sulfites are consumed but which go dormant when sulfites are not eaten...so you may need to address bacterial imbalance in addition to the sulfite issue.
Unfortunately, probiotics create sulfites also, so you may need to start slowly with probiotics and try various different brands until you find one that works for you... I do well with Jarrow's Allergen Free Jarro-Dophilus despite my severe sulfite sensitivity.
And don't forget about the methylation connection - be sure you are getting adequate Vitamin B12 & Molybdenum, Magnesium, Zinc, Iron and using active forms of Folate (want methyl-folate, or 5-methyltetrahydrofolate and not Folic Acid) and B6 (want P5P vs. the usual pyridoxine HCl).