Be reassured that the feedback you got from your partner wasn't negative, but next time you should also be open with her about your feelings regarding what happened. Explain that it worried you and that her wellbeing is very important to you. I think the more you talk about it as a couple the more you will find understanding and reassurance from one another.
The technical term for the emotional state your partner experienced after your session is subspace. Subspace occurs when feelings of pain cause the body to be flooded with a variety of different chemicals, including adrenalin.
The body reacts to pain and other extreme stimuli by producing chemicals which can give you a buzz or high that many people associate with BDSM play. People who indulge in extreme sports also get the same sort of buzz, with similar after-effects, for the same reasons. When your body comes down off the high, the chemicals that flood your system can cause you to shake and shiver. The tears and fatigue that your partner experienced after previous sessions are also symptoms of subspace.
BDSM type play can be very therapeutic and provide a natural high, with a related release of emotions. I have often experienced this when my Dom and I have indulged in an intense period of play. The real key is to provide good aftercare. Discuss with your partner about the type of aftercare she would like best. Certainly staying with someone until they have recovered (as you describe doing), providing cuddles, somewhere warm, safe and comfortable to rest and perhaps something light to eat and drink are all gestures that can help your partner feel protected and relaxed after a session.
