Ooo, that's a keeper, Stella, thank you. Just started having that problem myself.
Karen, I hope it works for you tonight! (And yes, I'm constipated too, though all the fiber I'm eating does fight its way through now and then. TT and one parathyroid out on 7/18.)
It is a PH imbalance due to the hypothyroidism you may be having. If you are still under your optimal level of T3 you can suffer this symptom.
There is a natural PH balancer if you want to try it. Mixing 3T of apple cider vinager and 1t baking soda in a full glas of filtered water 2x's a day can work.
Your PH levels should be at 6.5 so you may need to get urine strips to test each morning.