I know you don't want to hear this but if you don't want to get pregnant please get on birth control or use condoms.
Plan B is called emergency contraceptive because its meant to be used in emergencies and is horrible for your body. In your post you've already used twice in one month and thats concerning.
Now having said that, plan b can cause your periods to be irregular for some time afterwards. If your concerned about pregnancy take a test, thats the only sure way to find out. Good luck