is she sick or depressed?

Question
hi, my 8 month old chocolate labrador puppy has been very lethargic since the clocks went forward. She has not been as keen to eat her food but does eat it and she seems to get tired out really quickly on walks. I took a bottle of water with me today and kept giving her drinks but didn't seem to make a difference. Have changed her food gradually to James wellbeloved from science plan and am wondering if she is down because of this. She really liked the science plan but it is so expensive. She doesn't have any diarrhoea or anything so i cant understand it.
The other thing is i cannot make myself interesting enough to stop her running after other dogs in the field. She is very friendly to all people and dogs that she meets but the little dogs dont much like her and their owners can get a bit irritable.
I now have her on a 30 ft trailing lead but she hates being on it. she doesn't run with it on and she is putting on weight so she needs to run!
i have tried balls and squeaky toys but even though she goes mad for these at home she is not a bit interested when in the field.
She will stay with me all the time if no other dogs but goes deaf and wont come to me at all if another dog is there.
I have treats on me all the time but these are no good at all in comparison to the other dogs.
Please help if you can.  

Answer
I am not familiar with that James Well Beloved chow.  I lack confidence in the smaller manufacturers.  They don't have the resources the larger companies do to assure good nutrition.  I think if you compare the label on Purina 1 and Science Diet, the biggest difference will be the price tag.  Both are fine chows, proven by use in large service dog programs.  Whatever chow you choose, stick to the lamb in the science and go with an adult chow.  That should slow her growth a little giving her joints more time to develop.  If it is putting on weight, evaluate the dog as illustrated in this link, http://www.longliveyourdog.com/twoplus/RateYourDog.aspx  You may want the vet to confirm your judgment. Adjust the dogs food and exercise as needed to reach its ideal body condition.

You may need to give her better leadership.  The key to most behavior problems is approaching things using the dog's natural instincts.  Dogs see all the people and dogs in the household as a pack with each having their own rank in the pack and a top dog.  Life is much easier if the 2 legged pack members outrank the 4 legged ones.  You can learn to play the role of top dog by reading some books or going to a good obedience class. A good obedience class or book is about you being top dog, not about rewarding standard commands with a treat. Start at http://www.dogsbestfriend.com/  For more on being top dog, see http://www.dogbreedinfo.com./topdogrules.htm

You might try to find other owners of young, large breed dogs and let her induldge in the rough play Labs seem to enjoy.  Perhaps then she won't be as quick to annoy the owners of smaller dogs.