when you say plan b do you mean the morning after pill?
I think she is right, you are spotting from the plan B. Did you take the plan B the day after or right then? well i hope all is well and good luck!
Most likely your spotting is from the plan b. It can make you spot between periods. There is a chance that it could be implantation bleeding but since you took plan b I seriously doubt it. When is your period due?