What can I do to stop her from urinating on my couch?

My female cat, approximately a year and a half old, urinates on my couch whenever my back is turned! If I am near the couch, she has no problem using the cat box (which is in the bathroom)! She also likes to use the cat box whenever I am in the bathroom! But every night, sometime after I have gone to bed (she sleeps on the bed with me), she goes in and pees on the couch! The only change in her life recently has been her comming into heat. This began this summer, and that is when she began urinating on the couch. I assumed it was a manifestation of this new cycle for her, but now she does it every day and night! Basically any time I am not in the room, or in the bathroom with her! I want to make it clear, I do not have to be in the bathroom for her to use her litter box. In fact she seems to be perfectly comfortable using her litter box all day long, it’s just that every time I go into the bathroom, she comes in and uses the litter box. I have tried cleaning the couch with a “green” cat urine cleaner, I have put shower curtains over the couch (thinking she would not like the sound of the plastic) and nothing I have done stops her! I hate to take her to a shelter, her young life must have been awful, because she is afraid of every human being! She has grown to trust me a little, but she does not purr, she does not sit on my lap, she does not like to be petted. She does want to lay next to me in bed, and once in a while she will tolerate my combing her for a minute or two. What can I do?!?