The brown spots you saw are considered ketamim (stains) as opposed to an actual flow which would automatically render you niddah.
Stains found on the bathtub may be disregarded and do not render you niddah as long as you did not actually see the stains exit your body. The stain on your leg may have rendered you niddah depending on the size and color of the stain. If there was less than a k'gris (the size of a US dime or Israeli shekel) it may be disregarded. Note that stains on the skin are measured cumulatively – if several small stains would add up to a k'gris, the stain is problematic. On other surfaces, each stain is measured individually.
Stains larger than a k'gris are evaluated by the shade of blood. If the brown stain was the same shade as the brown on your bedikah which was ruled acceptable, you may disregard the stain. If it was light brown (the color of coffee with milk) with no reddish hue, it may be disregarded as well. If it was a brown shade with a reddish hue you should assume it rendered you niddah.
If you are unsure about the size or shade of the stain, and you no longer have it to bring it for evaluation (i.e., you didn't transfer the stain from your skin to a white cloth), there is room for leniency to disregard the stain since it was found externally, and you were tehorah at the time.
Halachic authorities differ regarding the observance of onot while using hormonal contraception; you should follow the ruling of your rav.
Please feel free to get back to us with any further questions.