Me and my 2 children are on Methotrexate as well for the treatment of Arthritis. Our doctor has always suggested that we do not take it until we are feeling better as it weakens the immune system. Especially since you have had a low grade fever.
Good luck and take care!
Sounds like you picked up a bug that your body isn't used to. I've moved around a lot, and for the first year after every move I'm constantly sick until my body builds up immunity to the unfamiliar bugs. Every region has it's own! I'd go off the mtx until you're feeling better. If you get dehydrated, which is a danger if you're sick, the blood serum level of mtx could get too high. Definitely call your doctor. If you're in a country with universal health care, you may be able to walk into any clinic and get treated - I know i did when I got food poisoning while traveling in Ireland. The doctor gave me medication and took a blood test.
For future reference, whenever you travel it helps to take a cotton swab (q-tip) and coat the inside of your nose with a thin layer of petroleum jelly. It'll create a barrier between your nasal tissues and any airborne beasties you breathe in. And keep your sinuses comfortable in the dry airplane air.
Feel better soon!