My 6-year-old dog is suddenly tired. Is this normal for her age?

Hi and thanks for using Petco Pet Education Center, formerly Petcoach! Sorry to hear about Harvey. This is definitely not normal and you should have her evaluated immediately. If she is not spayed and has had a heat cycle within the last 30-60 days, she may be developing a uterine infection called a pyometra. This can be a life-threatening infection unless it's addressed with surgery. Spaying is the recommended treatment for this condition. The clinical signs include fever, lethargy, weakness, increased thirst, increased urination, inappetence, vomiting, etc. Sometimes there is abnormal discharge from the vulva if the cervix is open and allowing the infection to leak to the outside. Other things, of course, are possible but if she is not spayed, this would be my first guess. Your vet can gather a history and assess her physically. Possible tests would include blood/urine tests and imaging studies (x-rays/ultrasound). Good luck with Harvey.