Trans Girlfriend

My live in girlfriend is trans. A biological male that lives as a female. She is very attractive and passable. Has been living full time as a female for over 10 years. She is on hormones to maintain her female features. The hormones do mess with her ability to maintain get and erections. For that reason she prefers to play the 100% girl role.
But... Every couple of weeks she gets in a mood. Her doctor gives her a prescription to help out with that hard on issue. When I see her get out that bottle I know I am in for it. She starts popping them like candy and I spend the day sucking her cock and taking it in the ass. When she gets in this mood she is super horny and can't get enough. The first time it happened I could barely walk after and was sore for a week!