Based on the information provided it seem like an "INFECTION" of the nostril
mucosa (under covering).
It normally occurs due to frequent NOSE PICKING, leading a scratch by nails/ the hair pulling from nose. The inner skin has already presence of bacteria "STAPHYLOCOCCUS
EPIDERMIS" which is commonly lead to infection in case of any breech/ cut in the covering layer.There are chances that other bacterias may be present. The swelling is due to the inflammation reaction caused by body's immune system so that more blood can come in bringing more of white blood cell
(fighting cell) to deal with causative agent, which in your case is most probably the bacteria. Pain is the part of the same inflammation reaction.
Please contact physician as soon as possible, as chances are it may spread the infection to other parts.
here are the steps which your physician may take
1) Will take a physical look
2) may order culture for the out coming fluid
3) Prescribe antibiotics and pain medication to have fast relief
4) may need to change the prescribed antibiotics in case of culture comes positive to other bacterias
5) may need to drain the pus if causing pressure and leading to pain.
Hope it helps.
Dr. JAY PATEL