I strongly recommend going to see your doctor
. Taking Plan B can definitely interfere with your regular menstrual cycle and can lead to some heavy bleeding
, cramping, or menstrual irregularity. Plan B is also not 100% effective, so there is always still some risk of pregnancy even after taking Plan B. However, it does sound like you had a normal period after this, so that makes this less likely, as does the fact that you did use condoms.
However, if you have had any unprotected sex since (this is not clear from your question) then you could have conceived at a later date, since the timeline you are talking about here is more than 1 month and could potentially encompass two of your cycles.
Your doctor will be able to perform a pregnancy test. If this is negative, they will be able to help you figure out what else might be going on, which could include simply some menstrual irregularity from use of Plan B. Importantly, they will be able to help you think about whether another more reliable form of contraception
might be helpful to you, so that you do not have to feel that you need to use Plan B!