Surprisingly, give your teeth a very thorough brushing.
A tiny amount of food lodged between your teeth can cause gum pain probably because it can force the teeth very slightly apart, putting pressure on the gum.
If you haven't any mouth wash I would have a good swish around with some pre boiled (but now luke warm) water with salt added. Helps if the gum is inflamed because of infection. Bonella is good, so is oil of clove from Boots which is quite cheap, doesn't smell that nice but does the job.