You could consider sending her a letter threatining court action... might get her to concentrate.

Given the nature of the agreement (your word against hers) I probably wouldn't go as far as court, although you don't have an overly bad case, there's no proof if she lies.

If she tells the truth:

Judge: Did your bitch mate with xx's stud dog?
Her: Yes it did
Judge: Have you paid her?
Her: No

you see where I'm going, however if she says no, or decides she heard differently on the agreement, you're royally screwed and you'd most likely lose.

I'd definately be sending her a strongly worded letter though, and cross her off your xmas card list