What Causes Irregular Periods?

Hi! My name is Bev. My menstraul cycle has always been regular (28 days). However, it doesn't seem to be that way now. My period began on March 14, 2011 at its regular scheduled time. It ended on March 18th and I had sex (unfortunately, unprotected) on March 19th. Immediately after intercourse, I took the Next Choice plan B pill. On March 24th, I started spotting but it did not appear to be a regular period type blood, it was brownish red like. Later on that day the spotting stopped and I thought I was fine. Well, the next day, I woke and saw no blood. Around 10:00 am that day, I started spotting again but it got heavier to the point I needed a regular pad and experienced all the symptoms of a regular menstrual cycle. Anyways, I ruled out implantation bleed and a possible miscarriage because I realize that I was having  my period again after only 6 days. 

I thought this was a little weird because it had never happened before, but I thought that because I took the plan b pill, my cycle changed. I got my next period on April 14th and it ended on the 18th. Usually, I calculate when my next period is scheduled to come based on the length of my last cycle and when I am likely to ovulate and be most fertile. My thought was that since my cycle started March 24th and began again on April 14th, the length of my cycle was now 21 days and I am likely to be fertile between April 20 thru April 28th. Based on this, my next cycle should have began on May 5th. Well, it hasn't and my partner and I had intercourse on April 26th, unprotected (I did not take or use any contraceptives). 

Now, I missed my period; I am experiences some early pregnancy signs; I took a home pregnancy test today, it was negative. I just don't know what to think or how I feel. I'm not sure if my cycle is just going back to 28 days, as opposed to 21 days? I don't even now if I even ovulate now because of the irregularity of my cycle. 

Any advice, suggestion, or whatever would help me at this point.