My period and the plan B pill

Last month I had sex with my bf about 3 days before my period was meant to start and I took the plan B later the next day. My period came as expected and I thought that all was well. Until this months period and I know that the pill can mess up the next few periods but it didn't my first one. And it's been 11 days now since my missed period. I took 2 pregnancy tests Wednesday and both came out negative and I guess I did it wrong because I didn't take one after the other and I took them when I actually had to pee. Someone I know told me I did it wrong but I don't know... Also I look like I gained a little weight but I weigh myself and its normal weight. Should I try another test?