Blood found on a sponge, like that found on a bedikah cloth, is treated stringently because the sponge was inserted internally.
Therefore, you may not rely on your doctor's statement from a few weeks ago that she saw cervical bleeding. You need to return to your doctor or a bodeket taharah ("niddah nurse") to be examined. If a cervical/vaginal wound or irritation is found, you may attribute the blood on the sponge to that source. If there is no visible wound or irritation, unfortunately you would be rendered niddah from the blood you found. You would need to wait five days from the time you found the blood before counting seven clean days and immersing in the mikveh.
You are not required to look for any staining on the sponge when you remove it. Therefore, in the future, do not look at the sponge when you remove it. Discard it immediately, and wash your hands right away without looking.
Please feel free to get back to us with any further questions.