Not sure where exactly to post this. This is the second time I've had this happen. Situation- Shuttle is in nearly circular orbit of 252.2 by 251.7. Do EVA with one of two astronauts. After astronaut re-enters spacecraft, I shift control from astronaut back to command chip. Immediately I see my orbit has changed from 252.2 by 251.7 to 388. 6 by 138.4. After astronaut re-entered and before I shifted control, the orbit was still circular. My plan was then to fast forward to apiapsis and bring my periapsis up to 250. While in fast forward my orbit's apiapsis was increasing and periapsis was decreasing. I exited the flight at that point. The shift of control from astronaut back to command chip seems to be the trigger for this bug.
@Kell Yes, throttle was zero.