I've had the same experience lately. I've always been the non-athletic type - I got Cs in PE and As in everything else - always the last kid to be picked for any team activity. I did join a gym and greatly benefited from the water aerobics class 8 years ago.
A couple of weeks ago, I decided to try doing a simple yoga routine each day in my own living room. I actually did it for 5 days straight, much to the amusement of the two cats. By that weekend, I felt the same way that you just described - so sore and fatigued that I concluded that the exercise just wasn't worth it. I can live with the 15 extra pounds.
Fast forward to my PT appointment yesterday. The therapist showed me some very simple back strengthening exercises and had me to do them on the table. Yesterday evening, I was talking to my daughter on the phone, and I couldn't sit still in the chair - my back was hurting so bad. It continued to hurt most of the night.
I also have the same problems with my thigh muscles. Sometimes they cramp up or just give out, too. I'm a really funny sight getting up off of the couch. My doc keeps thinking that I might have degenerative disc problems causing this, so I had an MRI last week. I had knee surgery for Avascular Necrosis last year, and the PT thinks that the AVN might still be there, so it might be causing my muscle problems.
Another interesting thing that I've been reading is that lots of people with Sjogren's have muscle weakness and neuropathy (I have numbness from knees to toes). So there are three different theories for me.
I'll let you know what I ever find out about it.
At least we both know that we're not the only one!