Missed period

So last month if had my period 3 times because I had to take a plan b which made me miss my period this month. So I went to the hospital to check if anythings wrong they did a pregnancy test and im negative. I decided to go on the pill that day and this is day 2 of me being on the pill and I feel naseus to the point where I want to throw up can someone tell me what's wrong , please