A break in habit = sick????

Our family dog, Corgi, that we've only had about 2 months has started peeing on the carpet. She's 4-5 yrs old and until now has behaved as if she was house broken. At first, as long as she was let out regularly, she did her business outside. Now the "accidents" are more and more frequent to becomming every day. Just this evening she came in from outside and peed on the carpet about a foot away from where my daughter was sitting on the floor.

I, of course, got angry about it, but just chastized her and put her outside. I called my brother, who brought us the doggie, and he suggested maybe she was sick...possibly with a bladder infection. Anyone have any experience with this????