25 days ago my boyfriend ejaculated on my vagina. 4 days later I had a normal period. 7 days ago I started having light cramping and tender breast. I have taken a few at home test which all say negative. Im not tired or sick. Do you think it's possible I'm pregnant and the test isn't showing it yet?

Highly UNLIKELY.
You really should read up on how the whole process works.
SexEd 101: Pregnancy Symptoms - How Do You Know If You Are Pregnant? (http://www.sex-ed101.org/articles/pregnancy/pregnancy_signs.html)

What you did is called having sex, pregnancy can happen if he ejaculated or not.

While it is possible, it is not likely, Wait till your next period and see if you have it or not.