#1
I exercise my fretting hand with this for 10 minutes a day on my way to work. After work at night at about midnight, I practice guitar.

Am I overdoing it? I make sure I stretch my fingers before and after exercising or playing guitar. Should there be a day I don't do the hand exercises? Or stop playing guitar for a day as well?
#2
10 mins a day is fine. Although you can overdo exercises and get tendonitis, just stop doing it if you feel pain though.
GENERATION 10: The first time you see this, copy it into your sig on any forum and add 1 to the generation. Social experiment.
#3
its possible but 10 minutes probably wouldnt cause it, (guitar is known to cause aurtheritis) afterall so maybe thats the case? do they feel bruised or just overused?