[Tophersmom]

I'm not an expert by any means...but perhaps it's habit?

Are you using the same bedding on your bed that you're using on the sofa bed? Perhaps if you take the bedding from the sofa bed and put it on your bed. (?) She might be smelling her urine (even though you clean it) on the bed.
Are you using fabric softener? I know my previous dog had a real problem with fabric softeners of any kind (it's not recommended that you use fabric softeners around dogs, anyway)

Like I said, I'm not an expert at all....I hope you're able to find the solution!