Help! Need advice

I start my period about 2 weeks ago lasted 7 days and then ended like any normal period. I am currently switching birth control starts that the sunday after my period ended. About 2 days after i start birth control i started bleeding again. It seems like a lot of blood to just be spotting. It will go from bleeding a lot with no cramps to no cramps and bleeding a lot. I have taken 2 pregancy test and both were negative. I dont know if i should be concered. If anyone has ever had this before pleaae let me know.