Presuming you feel she should be fit enough, it is important to try and separate this problem into two issues – medical and management. The former will clearly require some veterinary input. The latter, some thought and detective work on your part.
A large number of medical conditions can cause the loss of form that you describe.
Even in a young bitch, womb problems, such as endometritis, can result in a vague malaise that is difficult to diagnose. Often, affected bitches have a poor appetite, an increased thirst and occasionally they vomit. Any vaginal discharge should be reported to your vet. A non-invasive ultrasound scan, coupled with routine haematology, can be diagnostic.
Heart and respiratory conditions are usually indicated by coughing, breathlessness and exercise intolerance. Any coughing dog should be isolated and rested. Monitoring the resting respiratory rate is a good way to assess your dog.
After being at rest for 10 minutes, count the rate for 30 seconds and double it. Normal rates are around 20 or less. Any continuing increase deserves investigation, as does a persistent rate over 30. Heart issues can give a blue tinge to gums, while respiratory infections cause red congestion.
Complicated blood disorders, such as auto immune haemolytic anaemia, are not uncommon in spaniel breeds. Affected dogs have pale gums, sometimes with little haemorrhages called petechiae.