My Degus Started Biting? Or Nibbling Too Hard?

[rcrothers951]

Well, You see. I've had my 2 degus almost 3 months now.
Pablo and Marcello.
Marcello has always been more forthcoming, whilst Pablo has always been really fast and shy. He always stuck to the shadows.
So whilst Marcello was already climbing up my arms onto my shoulder. Pablo stuck to the wheel. So i decided to just let Marcello out in the ball whilst i spent more time with Pablo.

Now Pablo and Marcello both run to the front of the cage when i'm there, they both jump on my hand and arms.
BUT They had both started to bite? Nibble too hard?
Marcello isn't too bad. But Pablo just hurts me. He nibbles/bites my fingertips, wrists, and any bit of skin :/ He bit my fingertip once and it started to bleed around my fingernail :/

Anybody suggest what i can do to stop this? Or if this is normal degu behaviour. I don't want them biting me everytime i'm with them. D:

Thanks anyway.

x