I have severe arthritis of my hiip, I was doing exercises yesterday and today I can hardly walk, why?

Are you under superv. Are you doing exercises under supervision? You must be doing exercises under supervision of a therapist or following a doctors plan of treatment only!
You over did it. Most likely the arthritic joint developed some inflammation. This is not uncommon in someone with arthritis. If you can medically take an anti inflammatory medication such as motrin, this most likely can be helpful. If you use common sense you can exercise without the supervision of a therapist. Understand that if you do too much, you may have some discomfort.