My 3 month old has a red inflamed streak in the crease of her neck. We ruled out her shirt collar irritating her neck because its only on one side, so we think it may be a heat rash. Any suggestions on what it may be and how I might fix it?

We don't diagnose on AMHD because, quite frankly, I can't see the rash, and I don't know what your living conditions are.

Is it hot where you are, and your baby has been sweating? I would use some tried and tested approaches like any linament for baby rash, as you use on her bum if she has a diaper rash; probably something quite similar, plus put her in a night shirt that is not tight around her neck, would be my advice.