Excess weight can certainly pay a big toll.

Id get her into a vet as soon as possible to run some tests to see whether it is just overheating from the extra fat on her body, or something more sinister like heart problems.

I second the vet notion. You need to speak to them about getting him on a diet.

Please consult your vet! Does she exercise? Is she fine walking? The vet can check to make sure that her weight has not affected her health negatively, and then help you choose food for her, and a diet, and a plan. Good luck!

How old is the dog? Age may be a factor as well.