bdbl,

No real thoughts as such, but, a hint to prevent putting a bit of pressure on it on the follow through.

Consider having the left foot open slightly at address - 11 0'clock.

I do this - I have a bad lower back. It helps with my follow through.

Sorry I couldn't help you with a diagnosis.

R.I.C.E.R. - Rest, Ice, Compression, Elevation and Rehabilitation. This process for the first 24 hours. If the problem continues see a doctor and get a scan or similar.

Ice application 10 minutes on, 20 minutes off.