White substance from cavy?please help!!!

Take her to a good vet as soon as possible. It's impossible for us to know that is going on or to give you medical advice or a prognosis.

Ok, I completely agree with VJ, and if you're truly that concerned, definitely take her to the vet.
What you described sounds like maybe bladder sludge or calcium deposit, and could very well be from changing the pellet. It's not healthy to feed a mix pellet. A plain pellet is what they should be eating, something high quality like Oxbow. Mixed pellets have a lot of additives, and other things in them that aren't healthy and potential choking hazards. In fact, a lot of pellets on the market aren't that healthy.
If you head over to the Diet and Nutrition part of the forum, and read the stickies, you'll get an excellent idea of do's and don'ts with their diet.
This website will be very helpful, too. Guinea Lynx :: Health Tips and Guinea Lynx :: Pellets
Hope all is well.