Pigmentation on the belly can be caused by a few different things. It can be triggered by hormonal changes, and since she is in heat, that could have something to do with it. It would be best to check with your vet if it's something that you are really concerned about, since it could be any number of things or nothing at all, and none of it is able to be discerned just based on the information that can be given on a message board.
I would definitely have her spayed, though, as if it is hormal, the spaying will level out her hormones and prevent things like that from happening. More importantly, removing her uterus will also remove her chances of developing pyometra, which is a life-threatening infection of the uterus that affects a large percentage of unspayed females. It will also somewhat lower her chances of developing mammary cancer as she ages, although had she been spayed when she was still a puppy, it would have lowered her chances of mammary cancer to almost zero.
I don't think it's anything serious at all, and I think she has probably developed this to some extent every time she has come into heat, and this time you just happened to notice it.
Just on a hunch, let me ask, have you given her a bath? A friend of mine once had a mini-doxie who had discoloration on her belly and my friend got all worried about it, until she gave her a bath and it washed off because it was just dirt. The dog had been laying in dirt under the car and it didn't look at all like dirt on her belly, but that's exactly what it turned out to be.
Ghilly
Increased pigmentation on the exposed areas of a dog's skin are common in hot sunny weather. My dog acquires brown freckleson her belly in warmer sunnier weather. My other dog did too.
However I don't know for sure if this is what is happening with your dogs.
I hope Ghilly comes on soon, and sees this post. It is possible she might have a better idea. She has a lot of experience as a Vet Tech.
Our dog has the same thing but she has allergies. Our vet said it's from scratching.
I have a chihuahua about 5 yrs old she has been in heat for about two weeks and she has discoloration on either side of her stomach on the lower half the color is dark and only on both side. it does not seen to bother her when touched. Should I bee concerned about this. Thank you