Sorry to hear about this problem. Following head trauma of any sort if you are concerned at all, it is always best to speak with your doctor
to determine if you need any further testing. As we all know, there are many different types of problems that can arise. In your specific situation, it is difficult to say exactly, but there are many possible explanations. Let's review a few of the common ones. First, it is quite common to have some bleeding
and bruising under the skin after hitting any part of your body. In most areas, the surrounding fat does a good job of disguising the lump. There is obviously less fat in your forehead than in other areas of the body, and so it may be that you are simply visualizing the slowly resolving hematoma that was caused when you hit your head. These can calcify in some people, and last quite a long time. Other possibilities include common tumors of the head and neck that are often noticed incidentally after a trauma similar to what you describe. Finally, there are different tumors of the head and neck that can be related to infection or things that are much more serious. Please speak with your doctor.