Stains on the body are evaluated based on adding up the sizes of all stains found at a given time. Therefore, if the stains on your legs were cumulatively larger than a gris (the size of a US dime or Israeli shekel) and clearly reddish, unfortunately this did render you niddah–even if each individual stain was smaller than a gris. The usual minimum days and seven clean days apply.
If the stains were altogether less than a gris, you are not niddah. You may continue taking precautions against becoming niddah from any further staining by wearing colored underwear and waiting after urinating before wiping. As you suggest, using disposable pantyliners is a good idea, as it can help you avoid finding further stains on your body. Please see our pages on stains and toilet paper for more details.
If the spotting becomes heavy enough that you need an actual pad or a tampon to contain the bleeding, you should assume you are experiencing a flow and are rendered niddah.
If you are niddah, you may want to check in with your healthcare provider about whether and how you should adjust your birth control schedule.
Please feel free to get back to us with any further questions.
BHatzlacha!