sudden aggressive behavior towards older dog

[ccrain04]

Help
!
We have a 5 yr old, not yet fixed, female yorkie. We got her when she was two from another household who did not reflect any such issues when we got her. She is loving and fun, loves attention (demands it), and constantly kisses us. We have noticed a couple of times when a very young child would make a sudden loud move near her that she would go into "attack" mode, although we always had her under controld and nothing happened.

We dog sit for our daughters 14 yr old mixed breed female shepard/lab mix, who is the sweetest dog ever. Never a problem with her. The two dogs get along great and have for three years, with only a couple of minor fusses over a treat or toy belonging to Lucy (our Yorkie).

However, things changed over Christmas. Lucy went after Carly (60 lbs vs 6 lbs) for no apparent reason. They were pulled apart B$ any damage was done. Since that time Lucy has attacked Caly four more times, totally unprovoked by any anything....most of the time Carly is laying down bothering no one. Lucy seems tense each time Carly gets near us, and is constantly sniffing the air, which she has not done before. Lucy was in heat the two weeks before Christmas as well. However she has been in heat before when Carly was around and they were best friends. We have a 9 month old grandson and are concerned that this behavior will carry over to the baby. We are at a loss as to what to do. We plan to have her fixed in the next month and hope this will help but we don't know if this is the problem or not. Don't suggest putting her down....my wife would put me down beofre that happened.

Does anyone have any suggestions as to what caused this sudden change and what can be done about it?
Skip and Jennifer in N. Carolina