Several things. Pain that severe needs to be worked up by your doctor in person. He/she will listen to you, perform an examination and possibly order some tests to see what's wrong with you.
I have foot pain on the lateral side of my foot around the fourth and fifth metatarsals when I'm pushing off. Walking barefoot or on flip flops relieves it while wearing sneakers aggravates it. Could it be some kind of neuroma or stress fracture?
Yes. It could be either that you describe. You should have it examined and probably have an x-ray taken to rule out fracture.
Possibility. Lateral forefoot pain often is caused by "overload" which can produce stress reactions, stress fractures, joint irritation and nerve irritation.
Possibly. Neuromas, stress fracture, and capsuliitis are the more common complaints in that area of the foot. A podiatrist would easily be able to determine which of these or another condition you had. It is possible that you have less pain unsupported because that is the position your foot has collaped into and the better supported position is uncomfortable because of the injury. Foot pain is not normal.
Could be. Most likely capsulitis due to hypermobility, get an x-ray to rule out a stress fracture. Neuroma not usual in this area.
Been having foot pain - lateral aspect of the proximal fifth metatarsal shaft, for ~2.5 months now. Not tendonitis or a stress fracture. I have my orthopedist and physical therapist stumped. Worse with activity, a lot better with rest. Any ideas?
Mechanical pain. From your symptoms, it is not a tumor, an infection, a nerve injury, nor an inflammatory arthritis such as rheumatoid arthritis. It is mechanical pain and could still be a hidden fracture. An MRI may show the problem.
See below. If concerned of a fracture then you need to see a podiatrist for x-rays and evaluation.
Pain in 3/4th metatarsal of left foot? Especially bad after sitting for a while. Not as painful once I'm up and about. Could this be a stress fracture?
It could but. One would expect it to hurt more when walking then sitting. A neuroma is another possibility given the location you describe. A foot doctor based on an exam should be able to make the diagnosis. Xrays could help make the diagnosis.