or a muscle sprain ... does happen!!
erm, maybe you have an allergy to cetain things like your detergent, deoderant, perfume, shower gel etc. I have an allergy to latex so if using condoms it might be that. Or one of my exs used to have the same burning and he found (he wasnt circumcized) that when he pulled back the foreskin that he didnt hurt so much. maybe use a thrush cream or a preventive like biofem around the top to ease the burning, my current partner uses it regularly if weve had a bit of a session. but defo go back to you dr or gp and in the mean time take cranberry tablets as they'll help with urine problems if that.
hope something in that is useful