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First of all, let me tell you that there is no chance of pregnancy. Usually, ovulation occurs between the 12th to the 16th day of periods or better said as 14 days before the periods. Life of egg is 24 hours and that of sperm is 72 hours. So, there was no chance of egg and sperm meeting on the 24th day of the period as the egg would never have been alive for that long. So there was need of a pill at all in the first instance. I-pill is a high dose of progesterone hormone. The same hormone which is produced in your body in the second half of the period after ovulation. This pill will suddenly increase the thickness of the inner lining of the uterus making it unfavorable for a possible pregnancy to implant. As it was a single dose, it had no strength to stand on, so it fell off and you got periods or bleeding from the increased thickness within seven to 10 days of taking the pill. This bleeding would have been thick, in fleshy pieces, dark red or brown, with slight lower abdominal pain until the bleeding is really reduced.
So if you take one i-pill, it would make the endometrial lining unfavorable for an embryo to sit there, a second i-pill is never needed. Repeated use of i-pills in such short span will only disturb the hormonal balance and the subsequent periods. If at all the embryo had formed, which is not possible from the calculations you have given, then your urine would have been positive after 14 to 16 days of sex, which you said was negative. All the symptoms you have mentioned are due to the progesterone hormone. This is the i-pill and incidentally the same hormone that supports the pregnancy. So, just forget about it and relax.
If you feel that the periods are getting irregular and different in amount then I suggest you just have any of the birth control pills like tablet Ovral-G (Norgestrel and Ethinyl Estradiol) or Dear 21 (Ethinyl Estradiol and Levonorgestrel) or Novelon (Ethinyl Estradiol and Desogestrel) or Duoluton-L (Ethinyl Estradiol and Levonorgestrel) or Intimacy plus 0.03 (Ethinyl Estradiol and Desogestrel) one tab at night for 21 days.
I am advising it not for the contraceptive benefit but for its hormonal level correction. You can have it for 21 days and stop and then the next periods will be within four to seven days. Further cycles too will be regular. Consult your specialist doctor, discuss with him or her and take the medicines with their consent. I suggest you can have the same pills cyclically for further contraception. Hope I have clarified your query and do write back if you have any more queries.