Why am I experiencing monthly two period cycles?

If you have missed the periods for 10 days but still the pregnancy test is negative, then, the possibility is that you are having a hormonal imbalance or may be some other issues. It needs in person evaluation to proper diagnosis and get the right treatment for you.
 

The birth control pill is best if taken within three days of having unprotected sexual intercourse. The mechanism of action is to prevent the ovary from releasing an egg. Be mindful, though, the morning-after pill is not 100% effective. To be safe, you should look out for the signs like missed periods, nausea, or breast tenderness. If you get any of these symptoms, it's best to do a pregnancy test.

You might be experiencing early signs of pregnancy. Sometimes, if your period is late along with symptoms like nausea and clear discharge, it could indicate pregnancy. To be sure, take a home pregnancy test. If it’s negative, the delay could be due to stress or hormonal changes.

It is highly likely that you are not pregnant because you mentioned that the test was negative. Nipple pain could be attributed to hormonal changes, stress, or even caffeine. However, if you are still concerned, it would be wise to pay attention to your symptoms and periods. 

If Plan B was used afte­r ovulation, it could influence your period. Plan B's function is to postpone­ ovulation, which may affect timing. Therefore­, it's possible for period delays afte­r taking it. Irregular bleeding and cycle­ fluctuations are potential symptoms.