Thank you very much for the useful information.
Given the fact that this pain is of very short durations and started soon after riding --- the most likely possibility is of a muscle spasm.
Muscles (especially if they are flat and thin --- just like around the shoulder) need not to be painful on touch when this happens.
Recurrence of cancer and metastasis to bone would not give you localized pain, it should be widespread in all the bones. And honestly, with such a clear history (with evidence of trigger of pain) and for such a short duration i would not consider recurrence of cancer at this stage at all.
Continue with ibuprofen in better doses 400 mg thrice daily.
Add tylenol to ibuprofen if the pain does not settle alone with ibuprofen.
Application of heating pads (one of the best muscle relaxant).
All will help.
Rarely you may need a prescription based muscle relaxant like cyclobenzaprine or tizanidine.
Stay adequately hydrated while you use these medications.
And watchfully monitor your symptoms over next 72 hours by which they should improve a lot.
If the problem persist, does not improve or gets worse beyond that, see your doctor for a clinical examination and further evaluation.
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