You have been on birth control for three weeks and took the plan B- is that correct? It's really highly unlikely that you would be pregnant, there is always a slim chance of failure with any birth control method, but I really don't think that it's at all likely that has happened for you.
If you are worried about conceiving from sex that took place about a week ago- the tests wouldn't show up as positive yet. You can give it a few weeks and test again.
Let us know how you are doing.
Probably the plan B was overkill. But I guess better safe than sorry. My suggestion is to just give the doctor or clinic that prescribed the birth control a call and ask them what to expect. You added a big does of hormones when you were already regulating with hormones. This will affect you in some way such as side effects. When on the pill, you really don't get a 'period'. You take 3 weeks of active hormone and then a week of sugar pills. The sugar pills causes you to then bleed. You never ovulated though when on the pill so it's not like a real period. You are right though, you may not have that down time where you spot like you would if you were just taking a normal pill packet. I'd again, just call your doctor to confirm that. I would not think you got pregnant from the double whammy of 3 weeks of birth control (which is like a full cycle of active pills) and Plan B on top of that. good luck