I have this from time to time. Remember that blood flow increases to the pelvic area, and standing for long periods of time can cause that blood flow to be even stronger. Plus your baby is growing, and you may be feeling bladder pressure. It's worth a call, but I really don't think you have too much to be worried about, especially if it improves once you lay down for awhile. I'd be more concerned if it was constant and persistent and over a period of a few days.