What Causes Pain And Cramps On Left Side Of Stomach After Running?

I ran a lot yesterday and today im having pain on my left side of my stomach. Its mostly when i lay down on my left side. It hurts when i eat. Im getting a lot of stomach cramps. I dont have a themometer so i cant check my temp. I also have been BM's I have already went to the bathroom today and, this pain doesn't feel like i have to go. What could this be? Should i see a doctor? What should i try?