Have a read of this http://globaltherapies.wordpress.com.../shin-splints/
Missing one race isn't the end of the world. Sounds like a combination of changes has highlighted an issue that might have been waiting to pop up anyway. Best to get some rest in first instance, get booked in to see someone who can tell you what's wrong and advise you on rehab.
Need I say more?
wishing you a speedy recovery,
Lynne