We appreciate the sensitive nature of this question.
A stain on a woman’s finger after vaginal “splinting” such as you describe can make her
niddah. Because the finger is inserted internally, the leniencies of
staining size considerations do not apply. Her status typically depends on the color of the stain. This color can be evaluated by pressing the stain against a white cloth or tissue, which can be
shown to a halachic authority if necessary.
If we understand your case correctly, the
IUD is as likely to cause staining as
vaginal irritation. If that is so, you should not be lenient unless you can confirm through examination that you have a vaginal irritation that bleeds on contact.
What we recommend you do in the immediate future is to keep colored tissue by your side at the toilet (and in your purse) and wipe your finger off on the tissue and flush it, taking care not to look at it. Avoid looking again at the finger until after you have washed your hands.
Long term, this situation is less than ideal. Splinting can also lead to vaginal infections or more severe irritation. We recommend that you pursue treatment for the rectocele. Please consult your physician for options. Urogynecological physiotherapy can often be very effective in helping women with this condition.
Please write back with any further questions. We wish you a refuah sheleimah!