Its probably just a pimple or a small cyst, a collection of fluid under your skin or w/e. It doesn't sound like anything serious but just to be safe you should go see your doctor.
It is very silly to think that your doctor will be able to tell you smoked weed by examining the goop dripping out of your pimple. First of all, I don't think thats even possible lol. Second of all, your doctor is not allowed to disclose any information like that to ANYONE, unless it was something like absolutely detrimental to your health that your parents had to know about. Thirdly, you should always be truthful with your doctor if your drug use may be connected to your illness/injury. In this case, I think the pimple thing and the weed you smoked are
completely unrelated, and so you probably don't need to tell him anything, but in the past many times I've had to admit to my doctor the fact I used hard drugs regularly, so he could properly diagnose certain health problems. It sucks, but he will keep it between you and him.
I think you are just being a little paranoid because you know you did "something bad" and you are feeling a little nervous because of that. So just relax, go to the doctor, and stop worrying

I'm sure its nothing, but it would be good to find out. Also, I am like 100% sure that the marijuana and the lump are not related. I think the fact that it popped up a few days later is just a coincidence.