I recommend that you talk to your primary care doctor
about this issue at your earliest convenience. They can take a look and figure out what is going on and what you need to do.
When you hit your head hard, like you did a month ago, it is very common to have swelling and "bump" formation, which generally will then go away over a few days to weeks, as in your case. The fact that you now have another pimple-like spot is likely unrelated to that original injury
A tender, pimple-like spot is more likely to represent a minor skin infection. For example, folliculitis is a condition in which the base of the hair shafts become infected, leading to pain, swelling, and pus collection. Minor cases of folliculitis can usually be treated with good skin hygiene as well as topical antibacterial ointment.
However, it is always worth having these things checked out. Your primary care doctor can make sure that the infection is minor and does not require any more aggressive therapy, such as oral antibiotics or drainage of any potential pus collection.
Call and talk to your doctor as soon as you can. Good luck!