Thanks for the question and thanks for using JustAnswer.
I want to ask a couple follow up questions so I'm sure I understand your question.
Is your dog spayed? If not, when was she last in heat?
Does her stomach feel hard or tight or gassy or doughy?
How long has it been this way?
Let me know and we'll go from there.
OK, thanks for the extra information.
I think you do need to be worried. I think my initial worry when I read your question is even more of a worry since reading the extra information you gave.
It is possible your dog could have a pyometra. This is literally a uterus full of pus. It is common in female dogs that come in heat and don't get bred. And the timing is about right for it to be possible. It is a serious disease that can be life-threatening. Surgical treatment (i.e. spay) is necessary right away to prevent serious complications.
I think you need to take her in to her vet as soon as you can. Get her white blood cell count checked and have an x-ray +/- ultrasound of her belly performed.
Even if she does not have a pyometra, I think you need to be sure she does not have something else going on. No matter the cause, a distended belly that you can notice is not normal and indicates something is wrong.
I hope this makes sense. Good luck.