Sudden irregular cycles??

My periods are becoming more and more irregular. My last normal cycle was May 3rd-7th. I then had very brief unprotected sex (not even a few minutes) and my partner didn't ejaculate. It is possible that precum entered my vagina, however. 
My next period was late and I suspected I could be pregnant. I took a pregnancy test about a week after my period was late and it was negative. I took another a week later and it was also negative. During this time I had no bleeding or discharge of any kind.
I then had pinkish and brown spotting the day before my period began on June 8th. This period was late by at least 10 days and I never spot before my periods. This period ended June 11th. A few days after that I took another pregnancy test and it was negative.
I was sure I wasn't pregnant, but my next flow expected on July 7th came on July 26th. 20 days late!! It was a very heavy period, and it lasted a normal time. It was like my usual periods but it was just so late!  
I never had any pregnancy symptoms and my body hasn't changed in any way. I don't feel any pain, either. I don't feel pregnant but the possibility bothers me since I am not ready to have a child. 
It's very unlikely that I am pregnant, and if this period is late I am going to schedule an appointment with a gyno. I haven't already because my tests are negative and I'm not experiencing any problems besides late periods. 
I just hope someone can give me some tips or share their experiences so I can get a better idea about what is going on with me. 
-Rachel