Older Dog attacking

[highflyer]

We have a female WCS who is 9 years old now who has always lived with two other WCS one boy the other a girl unfortunately we lost the other two last year so decided to get a pup we rescued a 7 month old from a breeder we have had her 10 months now with no issues but suddenly the older girl has started to be really aggressive towards the younger girl and followers her around looking for trouble we now have to separate them, instead of being a family in the evening we have to put one of the dogs in another room, what can we do ?
When we go for walks on the lead the older dog tries to attack the younger dog on lead but when off lead they seem to be ok, can't carry on like this any suggestions please

[highflyer]

Thanks for the reply's
Sorry for the abbreviation yes WCS Working Cocker Spaniel
All 3 are female and all intact
The baby in the group who is 4 months old tries to get in between the other two to stop them fighting she doesn't understand
Amber the 9 year old who kicks it all off is due her booster this week so will enquire about the thyroid thing
Thanks again