This is Dr. Christian. Welcome to JustAnswer. A few questions will help me get a clearer picture of this situation:
Is it covered with hair or bare?
While an infection or insect sting is possible (typically there would be some drainage, bleeding or pain), it is also possible this could be a skin mass that needs veterinary attention. There is no harm in trying some medications and treatment at home for a day or two more but if it does not improve then have it checked. Here are some things to try:
If you have more questions please let me know.