Jana,
3 things come to mind when a patient describes pain in this area.
1. Stress Facture - stress fractures do not show up on xray for 14 days after the injury. Depending upon who you talk to, cortisone decreases the chance of a fracture from healing because it is attempting to decrease inflammation. But, inflammation is part of the "normal" healing process of any fracture. Bone scan or sonogram will show the stress fracture.
2. Tendonitis of the peroneus brevis tendon (peroneal tendonitis). Attaches to the back/bottom of the 5th metatarsal bone (the long bone along the outside of your foot. The tendon runs from theat bone around the outside of your ankle and to the muscle on the outside of your leg.
3. What I call cuboid syndrome - closely associated with peroneal tendonitis. Their are actually 2 peroneal tendons - longus and brevis. They run together from the outside of the leg and around the ankle then go their separate ways: one to the 5th metatarsal bone and the other under the "cuboid" to the inside fo the arch.
2 and 3 occur in golfers frequently.
It occurs in the target side foot because of the twist that foot undergoes from impact to finish. It can occur in either foot with a very minor twist of the ankle while walking/tripping on a slope or a hole (unless you play somewhere where all the fairways are perfectly flat and no elevated greens

)
In my experience, peroneal tendonitis does not go away by itself. Most of my patients go to physical therapy, and depending on the foot type will receive other treatments in the office.
Stress fracture of the 5th metatarsal can and frequently does get worse - turns into a full fledged fracture. Depending on its location in that bone, can require a cast/screw/or a plate with many screws.
With that said, MRI could be an option to give you the full range of possible diagnoses - tendonitis, joint inflammation, fracture, muscle strain.
Heat will frequently give each of these diagnoses relief initially. Tendonitis needs heat in the morning to "wake up/warm up" the tendon to decrease further injury, but ice for the rest of the day 10 minute intervals per hour.
I hope some of that helps. If you give me your location, I might be able to recommend a podiatrist or sports medicine foot and ankle orthopedist. They are not all created equal within or between professions.
Good Luck.