Spotting before period

Hi, I just had a question to see if anyone could help me with what could possibly be going on with my period. My last normal period was on June 20 2006 it lasted until June 24, which is normal for me. I started a new pack of orthotricyclen lo on June 25th and missed a pill on 26th so I took two on June 27th. On June 28th I came in contact with semen. On July 12th I started having a very light brown spotting which had increased to light red on July 13th this continued until July 19th. On July 18th I should have started my period which should have lasted the usual 5 days but only spotted for 2 days instead, and then there was nothing. I have been experiencing some nausea, fatigue, lower back pain, severe headaches, mood swings, and oddly stretch marks on my upper thighs, hips, and breasts (that could be from weight flucataion, but I thought I would include it anyway) tender breasts, extremely visable veins in breasts and upper thighs near groin. I took 2 hpts both were negative and today July 31 I had blood test for pregnancy which was also negative. If anyone has any clues to what could be going on I would really appreciate your input. Oh, could any of this be caused by stress?

Thank you,
KMS