Preterm Labor? Helpp!

I've been having cramps since about one o'clock this afternoon, it is now nearing 7 pm. I feel like I have a bowling ball between my legs and my have been having Braxton Hicks and back pains really consistently for about 6 hours. They hurt to the point I can't talk through them. I thought I was dehydrated so I have drank 4 almost 5, 32 oz cups of water, and no improvement. I went to the bathroom and what I'm hoping was pee, just right out of me like Niagara falls. Am I in preterm labor? My OB has no after hours number, do I need to call L&D. I also have so much pressure on my pelvis it feels like my vaginas gonna fall out.