This is not the typical presentation of a brain tumor- brain tumors generally present with things like headache, seizure, vision/hearing problems, nausea/vomiting, muscle weakness, behavioral or cognitive changes, balance problems.
There are tumors (malignant and benign) that can form in the bones of the skull, but most will have other symptoms than just the mass. There are some tumors that can present with just a bony mass and no pain (eg multiple myeloma and osteosarcoma)- but I think this is unlikely. Because you feel the bumps on both sides, it is very likely that this is just the shape of your skull. The skull has bony irregularities on the back that we often don't notice because it's generally covered with hair.
If the lumps are getting bigger, become painful, or you are getting any other symptoms I mentioned above, or fevers, night sweats, new onset fatigue, unintentional weight loss, or anything else that concerns you, have your doctor take a look. If there is any concern, you can have a CT or MRI.
Hope this helps.