From the symptoms that you have mentioned I concur that you might have a superimposed sub clinical viral infection with your crohns.
As you might know crohns is a type of inflammatory bowel disease which can cause immuno deficient condition and hence increase chances of clinical and sub clinical infections. The painful red lumps are erythema nodusum which are a commonly seen manifestation of crohns.
I suggest you do the following things.
1) The viral infection will subside by itself but as you are undergoing treatment with infliximab, I would suggest you take PPD test and continue yearly PPD test to rule out tuberculosis re-activation.
2) I would suggest baby Aspirin 325 mg for the erythema nodusum on your shin and arthritis.
3) Cold compress, leg elevation might also help with your erythema nodusum.
4) Its imperative that you maintain a good, healthy and balanced diet .As you rightly mentioned, blood urea is low because of reduced intake of food.
5) Get a colonoscopy every 3 years to rule out colon cancer.
If the symptoms persist for more than 2 weeks in spite of my recommendations, kindly consult a gastroenterologist who might schedule you for a colonoscopy.