late period

About a month or so ago right after my period ended my boyfriend and I were having sex.. he pulled out and was positive he made it in time but bought me plan b just to be sure. I took it maybe like 3-4 hours later that day. I was supposed to get my period 3 days ago and it still hasn't come yet.. but I've also been really sick the last 3 days and earlier this month the dosage on the medicines I take have changed and I stopped taking one. I also took a pregnancy test about 2 weeks ago and it came back negative. Do you think I should take another one just to be sure or do you think my periods late because I've been sick and the medicines? I'm also not experiencing any like pms symptoms or anything..