Dog decided to pee on the couch.. why?

well Tonight we were just sitting in the living room and my dog jumped up on the couch and snuggled up against me. (she is not normally aloud on the furniture) but it was so cute i just let her sit with me, she stayed like that for a few minutes then got up. i thought she was getting off or just re positioning. then she started digging at the couch and then she peed! why?! she was totally calm then she just peed! why did she pee? she also pees when she gets excited. but she was not at all excited. she was completely calm.