Oops not posted for a bit but need some advice

Generally I run slowly but last sat I dud sone fast running
Was sore for next couple of days
Went back out
Started to go fast

And whey hey STOP !!
My right thigh was in agony
No running even slowly

TODAYI STOOD up On THE BED on my  right leg
And have NEVER EVER HURT so much !!