Pot here refers to what you might know as pottie, chamber pot, chimmy, jordan, john, poe. I grew up with a plain white enamel chimmy with the pretty red flowers on the side. The older folks used a larger variety or sometimes a pail. These were essential amenities, especially at night, as the latrine was a good several feet to the back of the house. Modern life has pretty much done away with all that but in rural areas you will still find a plastic pot underneath many a bed.
The old time saying piss in the pot or get up is a lil old lady's way of saying you should make up your mind, be decisive, take action, do not vacillate.