It is not uncommon for a woman with even the most regular cycles to have stange symptoms at times.  There are many reasons for it.  Hormone issues can bring it on which would also explain the tender breasts.  You may have an issue this month and everything will be back to normal by next cycle.  With that being said, a pregnancy test is a good idea.  The early ones are very accurate now.  Sometimes they can detect a pregnancy as early as 5 days before your period should begin.

Light spotting in early pregnancy is commom with something called implantation bleeding.  I had it with my second baby.  It scared me but the doctor assured me that all was well.

 It could be pregnancy or may be period symptoms.  Get an "early midstream pregnancy test".  You can do these around the time your period is due.  Good luck - I hope the results are what you want.

you may be pregnant but sometimes prementrual symptoms can mimick early pregnancy symptoms.  And sometimes we can convince ourselves that we have pregnancy symptoms when we are not pregnant (I spent long enough trying to get pregnant to work this one out and its cruel).  You need to count your cycle from the first day of your period, not the day that your period ends.  Hence if it has been 21 days since you finished your period, add to that the length of time you had you period and that will be what day you are at this cycle (ie if you had a 6 day period, then you are at day 27 of your cycle).  you ovulate 14 days prior to your period starting, so if this matches up with your baby making attempt, then you may be pregnant.   good luck to you and possible congratulations.