Last year I got injured training for a marathon and was not able to run for about 6 months. I am finally better and started running again in february. However this year my monthly periods have caused a big problem. Every week right before my period I experience alot of pain either in my knees or a little lower than my stomach. Sometimes the pain are like sharp stabbing pains. Because of this I have missed alot of long runs. The longest I have ran is 12 miles. But now it is time for the taper! Should I do a 3 week taper or should I try to get in another long run? How many miles should I do on the taper? Has anyone else experienced problems with their period and running?

Hello!
What has happened to you? How did you injure?
Maybe you need to go to doctor before the marathon, just to be sure ...

I'm interested in preparing for marathon in 3 - 4 weeks.
That's very short time. I'haven't run 42 km before.
I can run only 18-20 km. (Haven't injured before.)
I need advice too... :)

You'll probably not like the answer but if you've only got to 12mls during your training I don't believe you're ready for this marathon.
You will get to 13.1, probably even 16mls but I'd suggest after that you're in real trouble. Maybe take this as a learning exercise and get you're mileage up to 18/20mls before going to a full marathon start line.