I believe it's the ortho low that is causing light periods. Especially since you just switched your birth control pills. Ortho low is notorious for light periods & spotting between cycles. I don't believe you are pregnant.
I too had very very sore boobs during my period when I was on Orthotricyclen.
With all of the negative hpt's, you are not pg.
I am not sure about the light periods but it wouldn't surprise me in the least if it was indeed due to Ortho Lo.