I'm Sorry If This Is Confusing, So I'm Sorry In

I'm sorry if this is confusing, so I'm sorry in advanced.
I am 20 years old, and have a active sex life. I started my period in 8th grade on the 15th... and up until a couple months ago my period has not been normal In April I had my period on the 15th for about 4-5 days.
Then May came along and I didn't have a period at all.
June 17th roles around and I finally get my period for 4-5 days.
Its now July 23rd and NO PERIOD!

Threwout these Ive tool pregnancy tests and they've all been negative. I just don't understand why I'm skipping periods, and randomly getting them because since my sex life is active I would hate to be pregnant and miss my oppertunitites at missing my pre-vitimins, exc. because after all the first trimester is the most important!

So my question is... What could be causing this? & Once someone has unprotected sex how many days does it take to read positive on a test?