Does Plan B not work for women over 200 pounds?

The Manufacturer's advice is that Plan B may not help if you are over 150lbs.
In that situation you'd need the other emergency pill, called Ella (Ulipristal Acetate).

I'm unsure what you meant by your fertile period, & don't want to assume the terminology, in case I give you wrong advice.

If you meant you were 3 days from your period, then, if your periods are regular each month, then you did not need emergency contraception, as you are not fertile 3 days before your period. If your periods are irregular then you would still need emergency contraception, as you would not know for certain when your period would start.

If you meant you were 3 days before the start of your fertile days/window/time of your menstrual cycle, then Plan B might help, but pharmacists would recommend Ella because of your weight.

If you meant you were 3 days before ovulation, then certainly pharmacists here in the UK would recommend Ella, even if you weren't over the weight limit for Plan B (Levonorgestrel), as ot is the only emergency contraceptive pill proven can prevent pregnancy during the 24 hours around ovulation, & unless you chart your fertility symptoms each day, or use an ovulation test, apps & websites are just guides as to when exactly you are fertile. They may not be exact.

As you've already used Plan B, it is not advisable to use Ella, even if still within 5 days of intercourse, as they do not yet know if using the 2 so close together, could cause them to cancel each other out, as there are not yet enough studies. That could leave you without chance of either working to prevent pregnancy.

Your only other option, if you are still within 5 days/120 hours of the unprotected sex, is to have a Paragard IUD fitted by a doctor. That can prevent the pregnancy implanting.