Losing weight in her belly

I have a 10 yr old female who is a bit overweight in her belly. I recently was away for 4 weeks and she was home with my fiancé. He cared for her and 

my other cat. I noticed that her belly bulge is smaller since I returned but she also seems more active. She's been running and playing more. Her appetite has not changed nor have any other of her behaviors. I don't know if I should take her to the vet for a checkup and bloodwork?

If she hasn't been to the vet for a bit, you should take her for a senior exam at minimum. It doesn't sound like anything is wrong, per se. But, the senior exam will include bloodwork with thyroid, it would be good to at least get some baselines on your kitty.

As for her increased activity level....is the weather cooler? Did your fiancé increase her playtime, change her food, decrease her food? Any of these things could have resulted in weight loss....which isn't a bad thing. I wouldn't be worried about the increase in activity, especially if the weather is cooler, I know my kitties get more active this time of year too.