What would cause my dog to fall down as she is walking?

My dog has just today (7-10-08) began falling over as she is walking.  She will walk a little bit and the her back end just falls down.  My daughter was at home with her today and said she fell over a few times earlier.  Then, tonight, when my daughter  was walking through her bathroom, her dog was following her and fell down.  So, my daughter told me she didn't understand why Gypsy is falling down on the tile today but that she had fallen a few times.  So, I began looking for ticks and found the one tick on her ear.  Since it wasn't on a nerve area I don't know if that could be the problem.  The tick was a soft, fat tick but I couldn't see it very well because she has a lot of hair.  I felt all over her but I cannot find any other ticks.  Her nose is also dry.  But, her gums are pink, which is a good sign.  She is an inside dog so I don't think the heat would be the problem.  Does anyone have any suggestions?