Why is there an odor down there at times, and what can I use to stop it ?

By down there, I am going to assume we're talking about the vagina. It is natural for there to be an odor, especially on days where you may have heavy flow or are sweating from doing lots of walking or physical activity that could create more friction in your underwear area. The important thing is to distinguish between a normal odor and a bad odor that may be a sign of an infection. The bad odors include smells that are very fishy, or other symptoms like lots of discharge, burning or itching (see possible STIs and signs). If you have a bad odor, you should tell your doctor.

While there are products that claim to make your vagina smell better, most of those products can do bad things to your natural vaginal microbiota (yes! it's a whole ecosystem down there) which can actually lead to itching, burning, or infections in the future. You can try taking showers or baths more frequently to clean the vaginal area naturally. However, if you are concerned about an odor, you should talk to your doctor so they can better understand what is causing it and find ways to make it better without disturbing the microbiota.