Willow was spayed 3 weeks ago. All was fine at her check up. Now her tummy looks a tiny bit red and swollen
She is absolutely fine in herself. Should I take her back to thy vet or is that normal?

I've recently had a bitch spayed and would definitely say that red and swollen is not the norm.

Yes get her seen , did she have internal dissolvable stitches as well as there maybe some that arent dissolving and her body is trying to reject them. When I had my CS this happened and had to go back a couple of times to get them tweezered out and the wound cleaned up.

Yes she had those disolvable stitches. Last night there was a brown bit on her skin which I gently rubbed off. It looked like a stich. But there were none visible after the op. Very strange.

Red and swollen after an op is definitely something to be concerned about.  Infection can take hold quickly and is best dealt with swiftly.  I wouldn't wait and see about anything with my dogs that I would be visiting a doctor with for the rest of my family.

Took her the vet and she said it's nothing to worry about. I take my animals to the vet much quicker than i'd get myself to the doc!