Possible Glitch- Orbital Anamoly

I had my shuttle in an orbit of 220.8 by 219.6. I did a tethered EVA that lasted maybe 5 or 6 minutes. After the astronaut was safely tucked back into the cockpit and the hatch (cargo bay) was closed I fast forwarded a couple of times to get to a planned burn to raise my orbit to around 250 by 219. Later I would circularize the orbit. I had my throttle set to zero and suddenly I saw my orbit was 440 by 45. I thought WTH because I had not done any engine burns. I thought well OK I'll de-orbit . I set up a burn to de-orbit and then I set fast forward. When I did that, the spacecraft accelerated and went on a parabolic path away from Droo. I have no idea why that happened. I know that on some EVA's I've done in the past, the orbit would change by a few KM, but nothing like what happened tonight.