I have been suffering with sore erect nipples and very tender boobs
For almost 3 weeks what could this be caused with

If your period has been delayed or you feel more tired than normal please take a pregnancy test. This will rule out a bad case of the pregnant for now. But if you still don't get your period then go to your doctor and have him take a look around and see what the problem is. He will do a blood test to see if you've come down with a case of the pregnant. Also, if you feel more tired and have to sleep a lot, be aware that the pregnant can definitely do this to you.