I have pain in my lower abdomen. It started after I run 10 miles whe I haven't run for a while. Whai?

Wait & See. Having some pain when you resume exercising after a break is not unusual. Best to start with shorter distances and gradually resume more strenuous activity. If it persists you should see your physician.
Muscle spasm. Lower abdominal pain after a 10 mile run may be muscle spasms from electrolyte imbalance. Keep yourself hydrated on long run and choose an electrolyte replacement drink without alot of artificial sweetner added.