Based on the information provided it seems as "INFECTIVE DIARRHEA" mostly an ingestion of some infected food.
It do not seem like another flare of Ulcerative Colitis
because you are taking medication for long time and its working very good for long time.
Form the diet it seems may be "Hamburger" OR " Lettuce /Tomato " have campylobacter/ E.coli bacteria and that in turn is the cause of diarrhea. But still many other possibilities as follows.
Following bacterias can be the cause food poisoning
2)E. coli enteritis
1)Campylobacter enteritis mostly seen in the meat products which are not cooked well (e.g. hamburger)
2)E. coli enteritis will be seen in Raw fish or oysters, or raw vegetables/fruits not washed properly.
3)Staphylococcus aureus is present in poultry product like eggs
4)Salmonella is seen in poultry product,
5)Shigella is seen in poor hygiene mostly water bourn
6)cholera is seen in water
based no the location / food it can change the causative agent.
ADVICE : CONSULT PHYSICIAN
Sooner the better to see doctor.
Here is what they will do
1) physical exam
2) ask history of food/ water intake
3) collect stool sample and blood test
for the culture
4) mean while will provide a broad spectrum antibiotic and ask you to keep hydrating yourself.
5) once the culture comes in they will check and may change the antibiotics if needed
Hope it helps.