Is it possible that the stain is from any other source (e.g., a cut from shaving, the towel was already stained, your husband had a cut and used the towel as well…)? If so you may disregard the stain.
However, if the towel was clean before you used it, and you saw the stain after wiping your body, your status depends on the size and color of the stain. Stains that are clearly reddish and larger than a gris (approximately the size of a US penny or Israeli shekel) would render you niddah. Stains smaller than a gris may be disregarded. Stains that are questionable in color, or borderline in size, should be brought to a halachic authority for evaluation. Please see our article on stains for more details.
If you need to ask a question about the stain but don’t have access to a halachic authority, we recommend Tahor App, which allows women to send accurate photographs for rabbinic evaluation. Tahor is currently available for iPhones and for some models of Android phones.
Please feel free to get back to us with any further questions.
BHatzlacha!